gst/gststructure.c: Fix fraction list fixation code. Take the fraction with the smallest difference with the target i...

Original commit message from CVS:
* gst/gststructure.c:
(gst_structure_fixate_field_nearest_fraction):
Fix fraction list fixation code. Take the fraction with the smallest
difference with the target instead of the first one in the list.
* tests/check/gst/gststructure.c: (GST_START_TEST),
(gst_structure_suite):
Added test to verify correct fraction list fixation behaviour.