Previously, URI.enscape could handle Unicode input (without any actual
escaped characters), or input with escaped characters (but no actual
Unicode characters) - not both.

    URI.unescape("\xe3\x83\x90")  # => "バ"
    URI.unescape("%E3%83%90")  # => "バ"
    URI.unescape("\xe3\x83\x90%E3%83%90")  # =>
                                         # Encoding::CompatibilityError

We need to let `gsub` handle this for us, and then force back to the
original encoding of the input.  The result String will be mangled if
the percent-encoded characters don't conform to the encoding of the
String itself, but that goes without saying.

Signed-off-by: Ashe Connor <ashe@kivikakk.ee>