Gst::QueryAllocation: fixed methods using AllocationParams objects

    * gstreamer/src/query.ccg: accessing to a gobject was done incorrect -
      not by gobj() method, but by dereferencing this object.
      AllocationParams holds pointer, to a GstAllocationParams, and not to
      a object, that's why previous solution didn't work fine.
    * tests/test-query.cc: added test which proves this fix.