Fix an incorrect autocorrect for Rails/ContentTag cop

[koic]

Follow up to #242.

This PR allows content_tag when the first argument is a variable because content_tag(name) is simpler rather than tag.public_send(name).

When public_send is used, it becomes complicated as follows.

First, content_tag(name) cannot be corrected to tag.name when the first argument is a variable.

So use public_send and prevent the following errors in that case:

Case 1: Using public_send to prevent the following NoMethodError

Original code:

content_tag(name, 'foo', class: 'bar')

Auto-corrected code (before):

tag(name, 'foo', class: 'bar')
#=> NoMethodError (undefined method `each_pair' for "foo":String)

Auto-corrected code (after):

tag.public_send(name, 'foo', class: 'bar')

Case 2: Using symbolize_keys to prevent the following ArgumentError

Original code:

content_tag(name, 'foo', {'class' => 'bar'})

Auto-corrected code (before):

tag.public_send(name, 'foo', {'class' => 'bar'})
#=> `ArgumentError (wrong number of arguments (given 3, expected 1..2))`

Auto-corrected code (after):

tag.public_send(name, 'foo', {'class' => 'bar'}.symbolize_keys)

The symbolize_keys may not be needed, but for safe auto-correction
it will be added if optional argument keys are not all symbols.

Case 3: Using o ? o.symbolize_keys : {} to prevent the following ArgumentError

Original code:

content_tag(name, 'foo', options)

Auto-corrected code (before):

When the third argument is nil.

options = nil
tag.public_send(name, 'foo', options)
#=> `ArgumentError (wrong number of arguments (given 3, expected 1..2))`

Auto-corrected code (after):

tag.public_send(name, 'foo', options ? options.symbolize_keys : {})

Guard with the empty hash in case the third argument is nil.

---

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• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

This is a draft PR. I'm investigating if there is a better code to auto-correct it.

[koic]

/cc @tabuchi0919

[koic]

This is the note that has just been decided. It takes allowing content_tag rather than using public_send when the first argument is a variable. Thank you for advising @amatsuda.

[koic]

I updated this PR to allow content_tag when the first argument is a variable, method, or constant because content_tag(name) is simpler rather than tag.public_send(name).