increase X3D compliance: don't encode urls at sight.

This seems superfluous to begin with. encodeURI() is neither
idempotent nor in any way sensible to do without specific reason. To
wit, jquery, the likely most sophisticated wrapper aroud XHR, simply
never calls encodeURI() anywhere whatsoever. I would support
force-breaking the build on occurrence - in generic code, this is
almost always concealing other problems.

I have not looked at possible changes to error handling, as some
errors might occur earlier which have previously been hidden by this
behaviour. However my cases (that can't work without this) do work. It
is ultimatly a user problem if the url is not an url, and  perhaps that
should be signaled clearly when setting the field.