rtph264pay: Change aggregate-mode default for RTSP at least

See !2 (comment 527142) for details.

In summary, aggregate-mode=zero-latency is breaking various proprietary NVRs while they worked in previous versions and with aggregate-mode=none. On the other hand Safari WebRTC doesn't work with no aggregation, which is why it was changed at all.

@ndufresne also says that aggregate-mode=none behaviour now matches live555.