MRI 2.1 updates

This updates the URI library according to https://github.com/ruby/ruby/tree/fc3c52eb37b145ebce2710c2cbf7662fcf0e378f. Note that MRI already adopted keyword arguments in the URI library (https://github.com/rubysl/rubysl-uri/blob/mri-21-import/lib/uri/common.rb#L968) so this can't be merged and released until Rbx supports keyword arguments.

I've also added some basic specs for `URI.decode_www_form` so we can at least test a tiny fraction of this method.