plugin: chain up set_context() vmethod

plugin: chain up set_context() vmethod

Since Gstreamer 1.7, set_context() vmethod needs to be chained up with
the parent class in order to broadcast all its contexts when the element
is added into a bin:

http://cgit.freedesktop.org/gstreamer/gstreamer/commit/?id=d5ded1588920c4471eefe055d09095d9e5e989b5

There is no need to guard the call, because before GStreamer 1.7, the
set_context() vmethod was NULL in the element class, hence the conditional
call make it safe.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=757598