polymorphic_path shouldn't modify options hash

Currently it's not possible to call first polymorphic_path and then polymorphic_url with the same options hash:

options = { :format => 'xml' }
polymorphic_path(record, options)
polymorphic_url(record, options) # => wrong result (path instead of URL)

This is because polymorphic_path changes the options hash to add ":routing_type => :path".