rtsp: RTSPLTransMap should default to multicast

This bug triggered by trying to connect rtspsrc to a session started
with VLC.

The command line to start the VLC RTSP server is:
cvlc \
  --loop \
  --sout="#rtp{dst=239.23.250.10,sdp=rtsp://:8600/Stream,ttl=5}" \
  --sout-all \
  --sout-keep TWICE\ \'Heart\ Shaker\'\ M_V-rRzxEiBLQCA.f137.mp4

Connecting to the client results in the VLC client returning the
following Transports in the SETUP reply
key: 'Transport', value: 'RTP/AVP/UDP;destination=239.23.250.10;port=5004-5005;ttl=5;mode=play'

GStreamer:
stream 0x7f457002cdc0 as UDP unicast

Since the multicast field is missing, GStreamer assumes unicast, while
in fact the default should be multicast.

On the other hand, when configuring the VLC server to send out unicast
(remove the dst in the command line), the Transport does contain the
unicast flag

'Transport', value: 'RTP/AVP/UDP;unicast;client_port=41390-41391;server_port=54344-54345;ssrc=7F296D19;mode=play'
32 jobs for bugfix/rtsp-transport-default-mcast in 43 minutes and 18 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #624748
gst indent

00:00:35

passed #624747
manifest

00:00:37

 
  Build
passed #634805
build cerbero cross-android universal

00:13:27

manual #624762
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #634804
build cerbero cross win32

00:11:09

passed #634803
build cerbero cross win64

00:06:26

passed #634802
build cerbero fedora x86_64

00:06:32

passed #634801
gst-macos-10.14
build cerbero macos x86_64

00:12:04

passed #624753
build clang fedora x86_64

00:07:46

passed #624749
build fedora x86_64

00:06:59

passed #624756
docker windows 1607
build msys2

00:12:10

passed #624750
build nodebug fedora x86_64

00:03:56

passed #624751
build static fedora x86_64

00:13:40

passed #624752
build static nodebug fedora x86_64

00:03:44

passed #624754
docker windows 1607
build vs2017 amd64

00:10:32

passed #624755
docker windows 1607
build vs2017 x86

00:10:31

failed #624758
build cerbero cross-android universal

00:01:03

failed #624759
build cerbero cross win32

00:01:57

failed #624760
build cerbero cross win64

00:01:03

failed #624757
build cerbero fedora x86_64

00:01:10

failed #624761
gst-macos-10.14
build cerbero macos x86_64

01:00:00

 
  Test
passed #634806
check fedora

00:04:11

passed #624764
integration testsuites fedora

00:14:54

passed #624768
valgrind bad

00:14:49

passed #624765
valgrind base

00:14:25

passed #624769
valgrind ges

00:13:16

passed #624766
valgrind good

00:15:01

passed #624767
valgrind ugly

00:02:13

failed #624763
check fedora

00:05:50

 
  Integrate
passed #624770
cross-android universal examples

00:07:03

manual #624772
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #624771
documentation

00:04:14