gst/gststructure.override: Properly check for gstvalue data types. This makes it possible to set fields with fractions.

Original commit message from CVS:
* gst/gststructure.override: (_wrap_gst_structure_set_value):
Properly check for gstvalue data types. This makes it possible to
set fields with fractions.
* testsuite/test_caps.py:
* testsuite/test_struct.py:
Test for above modifications