Extract ActionDispatch::Request#deep_munge

ActionDispatch::Request#deep_munge was introduced as a private method,
but was turned into a public one for the use of
ActionDispatch::ParamsParser.

I have extracted it into ActionDispatch::Request::Utils, so it does not
get mixed up with the Request public methods.
Merge pull request #10803 from gsamokovarov/extract-deep-munge

Extract ActionDispatch::Request#deep_munge