webrtcbin: Fix default value when SDP direction is unspecified
According to RFC 4566 (section 6), if sendrecv/sendonly/recvonly are not specified, the default is sendrecv, except for broadcast and H332 conference types.
According to RFC 4566 (section 6), if sendrecv/sendonly/recvonly are not specified, the default is sendrecv, except for broadcast and H332 conference types.