ephy-uri-helpers: Don't re-encode parameters ourselves

Implement Dan Winship's approach to detecting unwanted parameters
by reusing the original key pair values (without decoding/encoding)
when they are wanted, and simply dropping them when not.

See https://bugzilla.gnome.org/show_bug.cgi?id=724724#c14

https://bugzilla.gnome.org/show_bug.cgi?id=730464