requestMessage doesn't seem to match spec for 2 arity variant

Description

When exiting Neovim using vim-lsc, message "[lsc:Error] ElixirLS has crashed. See Output panel." is shown. Upon investigation, ElixirLS is throwing an exception (below).

I had originally opened this as #207, with the conclusion that it was the shutdown method being called out of order. However, after further investigation I noticed the requestMessage protocol definition may not be to spec. In the specification, the params is optional. However, in the code it seems to only expect the 3 arity variant: %{"id" => _, "method" => _, "params" => _, "jsonrpc" => "2.0"}.

Possibly related: #189

Steps to Reproduce

Open an Elixir file in a (Neo)vim instance using vim-lsc.
Ensure the ElixirLS is running.
Close the file.

Expected Behavior

The language server should exit cleanly.

Actual Behavior

ElixirLS crashes with the following:

21:16:59.997 [error] GenServer ElixirLS.LanguageServer.Server terminating
** (FunctionClauseError) no function clause matching in ElixirLS.LanguageServer.Server.handle_notification/2
    (language_server) lib/language_server/server.ex:216: ElixirLS.LanguageServer.Server.handle_notification(%{"id" => 2, "jsonrpc" => "2.0", "method" => "shutdown"}, %ElixirLS.LanguageServer.Server{analysis_ready?: false, awaiting_contracts: [], build_diagnostics: [], build_ref: nil, build_running?: false, client_capabilities: %{"textDocument" => %{"codeAction" => %{"codeActionLiteralSupport" => %{"codeActionKind" => %{"valueSet" => ["quickfix", "refactor", "source"]}}}, "completion" => %{"completionItem" => %{"snippetSupport" => false}}, "definition" => %{"dynamicRegistration" => false}, "signatureHelp" => %{"dynamicRegistration" => false}, "synchronization" => %{"didSave" => false, "willSave" => false, "willSaveWaitUntil" => false}}, "workspace" => %{"applyEdit" => true}}, dialyzer_diagnostics: [], dialyzer_sup: nil, needs_build?: false, project_dir: nil, received_shutdown?: false, requests: %{}, root_uri: "file:///Users/[redacted]/[project]" settings: nil, source_files: %{}})
    (language_server) lib/language_server/server.ex:144: ElixirLS.LanguageServer.Server.handle_cast/2
    (stdlib) gen_server.erl:637: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:711: :gen_server.handle_msg/6
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
Last message: {:"$gen_cast", {:receive_packet, %{"id" => 2, "jsonrpc" => "2.0", "method" => "shutdown"}}}
State: %ElixirLS.LanguageServer.Server{analysis_ready?: false, awaiting_contracts: [], build_diagnostics: [], build_ref: nil, build_running?: false, client_capabilities: %{"textDocument" => %{"codeAction" => %{"codeActionLiteralSupport" => %{"codeActionKind" => %{"valueSet" => ["quickfix", "refactor", "source"]}}}, "completion" => %{"completionItem" => %{"snippetSupport" => false}}, "definition" => %{"dynamicRegistration" => false}, "signatureHelp" => %{"dynamicRegistration" => false}, "synchronization" => %{"didSave" => false, "willSave" => false, "willSaveWaitUntil" => false}}, "workspace" => %{"applyEdit" => true}}, dialyzer_diagnostics: [], dialyzer_sup: nil, needs_build?: false, project_dir: nil, received_shutdown?: false, requests: %{}, root_uri: "file:///Users/[redacted]/[project]", settings: nil, source_files: %{}}

Environment

Elixir & Erlang versions (elixir --version):

Erlang/OTP 22 [erts-10.5.6] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]

Elixir 1.9.4 (compiled with Erlang/OTP 22)

Operating system:

MacOS Catalina - 10.15.3 (19D76)

Client (editor or IDE):

vim-lsc 0.4.0

NVIM v0.4.3
Build type: Release
LuaJIT 2.0.5
Compilation: /usr/local/Homebrew/Library/Homebrew/shims/mac/super/clang -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/config -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/src -I/usr/local/include -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/deps-build/include -I/usr/local/opt/gettext/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/src/nvim/auto -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/include
Compiled by brew@Catalina.local

Features: +acl +iconv +tui
See ":help feature-compile"

   system vimrc file: "$VIM/sysinit.vim"
  fall-back for $VIM: "/usr/local/Cellar/neovim/0.4.3/share/nvim"

Run :checkhealth for more info

JakeBecker / elixir-ls