check template UTF8 validity before parsing

[ggmichaelgo]

What are you trying to solve?

Liquid relies on regex for parsing, and Liquid needs to check the template's String encoding validity.

Currently, Liquid is raising a ArgumentError when a template includes an invalid UTF8 byte sequence.

require 'liquid'
Liquid::Template.parse("{% assign foo = '\xC0' %}") 

# lib/liquid/tokenizer.rb:31:in `split': invalid byte sequence in UTF-8 (ArgumentError)

Instead of throwing the ArgumentError, this PR updates Liquid to raise a Syntax error when a template has a invalid encoding.

With this change, the developers won't have to catch the invalid encoding error like this:

begin
  Liquid::Template.parse("\xC0")
rescue ArugmentError => e
  if e.message == "invalid byte sequence in UTF-8"
     ...
  else
    raise e
  end
end

Instead, the error can be handled like this

begin
  Liquid::Template.parse("\xC0")
rescue Liquid::Error
     ...
end