fix connect closed when Mint.HTTP.stream/2 errors

[parallel588]

hi. thanks for slipstream

I found one little error. I see the follow error message when connected is closed on server side.

[error] GenServer #PID<0.1985.0> terminating
** (KeyError) key :conn not found in: %Slipstream.Connection.Pipeline{built_events: [], events: [], message: nil, raw_message: {:tcp_closed, #Port<0.20>}, return: nil, state: %Mint.HTTP1{buffer: "", host: "localhost", mode: :active, port: 4001, private: %{extensions: [], sec_websocket_key: "ZuXVw5LkilQrAEZNeTpx7Q=="}, proxy_headers: [], request: nil, requests: {[], []}, scheme_as_string: "http", socket: #Port<0.20>, state: :closed, streaming_request: %{body: nil, connection: [], content_length: nil, data_buffer: [], headers_buffer: [], method: nil, ref: #Reference<0.1182401425.3777495043.38195>, state: {:stream_request, :identity}, status: nil, transfer_encoding: [], version: nil}, transport: Mint.Core.Transport.TCP}}
    (slipstream 0.0.0) lib/slipstream/connection/pipeline.ex:414: Slipstream.Connection.Pipeline.handle_message/1
    (slipstream 0.0.0) lib/slipstream/connection/pipeline.ex:35: anonymous fn/1 in Slipstream.Connection.Pipeline.handle/2
    (slipstream 0.0.0) lib/slipstream/connection/telemetry.ex:29: anonymous fn/2 in Slipstream.Connection.Telemetry.span/2
    (telemetry 1.0.0) /....../deps/telemetry/src/telemetry.erl:293: :telemetry.span/3
    (slipstream 0.0.0) lib/slipstream/connection/telemetry.ex:25: Slipstream.Connection.Telemetry.span/2
    (stdlib 3.15.2) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.15.2) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.15.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message: {:tcp_closed, #Port<0.20>}

[the-mikedavis]

this is published to hex now as v0.8.3 :+1: