Rename path_to_match to remaining_path

I think this is a better name, as it goes nicer with matched_path.
Don't override explicit nil :default_encoding template option in the render plugin

Some combinations of tilt/ruby have an issue where setting a
default encoding can lead to a frozen string error.  I don't
plan to work around the bug inside Roda, but one way of doing
so outside of Roda is to just not set a default_encoding, which
this allows you to do.