Don't report Trailing whitespace within a Heredoc

[Blacksmoke16]

I currently have a spec that is asserted against a heredoc that is failing linting due to there being trailing spaces in the heredoc (as the expected output also required them).

It probably makes sense to not apply the trailing whitespace rule for lines within a heredoc, given it's supposed to be interpreted as a multi line string literal.

E.g.

output = <<-OUTPUT

                
  Hello there!. 
                


OUTPUT