Allow for spaces in the heredoc tag

[jarretlavallee]

Currently, the parser will fail if there is a space in the heredoc tag. This PR updates the heredoc regex to allow for white spaces in the heredoc tag. With this PR, @("MYVAR" /) no longer causes the following error.

file contents:

class testing {
  $myvar = @("MYVAR" /)
       Some string
       in a heredoc
       | MYVAR
  notice($myvar)
}

error:

TypeError: no implicit conversion of nil into String
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/lexer.rb:496:in `initialize'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/lexer.rb:496:in `new'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/lexer.rb:496:in `interpolate_heredoc'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/lexer.rb:273:in `tokenise'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/checks.rb:25:in `load_data'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/checks.rb:55:in `run'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint.rb:205:in `run'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/bin.rb:66:in `block in run'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/bin.rb:62:in `each'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/lib/puppet-lint/bin.rb:62:in `run'
/Users/user/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/puppet-lint-2.3.6/bin/puppet-lint:7:in `<top (required)>'
/Users/user/.rbenv/versions/2.5.1/bin/puppet-lint:23:in `load'
/Users/user/.rbenv/versions/2.5.1/bin/puppet-lint:23:in `<main>'

[rodjek]

Thanks @jarretlavallee! Would you mind adding a test case to the lexer spec to ensure that we don't have an accidental regression at some point in the future?

[jarretlavallee]

@rodjek Sorry about that! I added a spec test and squashed the commits.