gsturi: Fixed incorrect escaping of path as a generic string

gsturi: Fixed incorrect escaping of path as a generic string

The gst_uri_construct function was escaping the location string
as a generic uri string. This is incorrect since the slash('/')
characters are reserved for use in this exact case. The patch
changes the escape_string function mode to handle the path correctly.

I have deleted the escape_string function since it is no longer being
used and have created a unit test for the function. I have also
deprecated this function in favour of the GstUri API.

https://bugzilla.gnome.org/show_bug.cgi?id=783787