[Fix #704] Add new Rails/StripHeredoc cop

[koic]

Closes #704.

Enforces the use of squiggly heredoc over strip_heredoc.

# bad
<<EOS.strip_heredoc
  some text
EOS

# bad
<<-EOS.strip_heredoc
  some text
EOS

# good
<<~EOS
  some text
EOS

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

Thanks!

Checked on a real-world repo:

$ rubocop --only Rails/StripHeredoc /Users/pirj/source/real-world-rspec/
57497 files inspected, 258 offenses detected, 258 offenses autocorrectable

All offences look legitimate.

[pirj]

Even though there's some stability in RESTRICT_ON_SEND, accoring to:

The on_send callback is the most used and can be optimized by restricting the acceptable method names with a constant RESTRICT_ON_SEND.

Do you think it makes sense to keep this line just to be on the safe side?

return unless node.method?(:strip_heredoc)

[pirj]

My suggestion might be overly paranoid.
I can't tell the exact share, but some cops seem to have this duplication, even when there's just one short method name:

https://github.com/rubocop/rubocop/blob/cdf0d4b9abbbefdff63aab9c87c74b20bd8ed8af/lib/rubocop/cop/style/array_join.rb#L27
lib/rubocop/cop/style/float_division.rb

I'm not aware of any practice regarding this. Please don't hesitate to ignore the above suggestion completely as you see fit.

[koic]

AFAIK, method? may be redundant with RESTRICT_ON_SEND because it only checks a method name. So, it is different from the def_node_matcher that checks other than a method name.