Add New AllowedKeys to Rails/HttpPositionalArguments

[steiley]

Related to #1489

Rails/HttpPositionalArguments is subject to correction of parameters for RSpec filters, so user is able to allow by key name in this change.

# AllowedKeys: [] (default)
#   # bad
#   # get :new, foo: 'bar'
# 
# AllowedKeys: ['foo'] 
#   # good
#   # get :new, foo: 'bar'

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• 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).
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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
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