Handle "@" prefix in from_data/1 events

[olegokunevych]

When using RTMP plugin with streams from some ip camera vendor, we got errors:

lib/membrane_rtmp_plugin/rtmp/messages/set_data_frame.ex in Membrane.RTMP.Messages.SetDataFrame.from_data/1 at line 51

lib/membrane_rtmp_plugin/rtmp/message_parser.ex in Membrane.RTMP.MessageParser.read_frame/3 at line 188
lib/membrane_rtmp_plugin/rtmp/message_parser.ex in Membrane.RTMP.MessageParser.handle_packet/2 at line 107
lib/membrane_rtmp_plugin/rtmp/message_parser.ex in Membrane.RTMP.MessageParser.parse_packet_messages/3 at line 68
lib/membrane_rtmp_plugin/rtmp_server/client_handler.ex in Membrane.RTMPServer.ClientHandler.handle_data/2 at line 168
gen_server.erl in :gen_server.try_handle_info/3 at line 2434
gen_server.erl in :gen_server.handle_msg/3 at line 2420
proc_lib.erl in :proc_lib.init_p_do_apply/3 at line 333

with following arguments: ["@setDataFrame", "@onMetaData", %{"framerate" => 25.0, "height" => 1080.0, "width" => 1920.0}]
Assuming that some vendors, doesn't follow RTMP spec, current PR should resolve the issue.

[varsill]

Hello @olegokunevych !
Out of curiousity - which IP camera vendor generates this non-spec-compliant bytestream? :D

[olegokunevych]

@varsill it's Dahua chinese vendor :)

[varsill]

I see, thanks for the contribution! I will release it in a moment