audiointerleave: Won't negotiate with alsasink

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#755228)

Description

Audio interleave in the case the channel-positions property is unset, will produce in caps channel-mask=0. This won't negotiate with alsasink (but works with pulsesink).

gst-launch-1.0 -v \
audiotestsrc ! i.sink_0 \
audiotestsrc ! i.sink_1 \
audiointerleave channel-positions-from-input=1 name=i ! alsasink

While with pulse it works:

gst-launch-1.0 -v \
audiotestsrc ! i.sink_0 \
audiotestsrc ! i.sink_1 \
audiointerleave channel-positions-from-input=1 name=i ! pulsesink