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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• Added tests.
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