When RSpec/MetadataStyle cop is used with EnforcedStyle: hash, offenses must be registered only for metadata arguments being symbols.

This code:

describe 'Something', a, b do
end

would previously report

describe 'Something', a, b do
                      ^ Use hash style for metadata.
end

And this code:

describe 'Something', a, :b do
end

would previously produce the following exception: "NoMethodError: undefined method `value' for an instance of RuboCop::AST::SendNode".

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).