Setting RTPSource::sdes cname rejects '@'

[Juan Navarro]

Trying to set the "sdes" property into a GstRtpSession instance, the 
command gst-launch-1.0 fails:

$ gst-launch-1.0 rtpsession name=r 
sdes="application/x-rtp-source-sdes,cname=(string)user at host" ...
WARNING: erroneous pipeline: could not set property "sdes" in element 
"r" to "application/x-rtp-source-sdes,cname=user at host"

The reason is that at some point, the '@' character is not recognized as 
a valid one.
These cnames work fine: userhost, user_host, user+host.
But user at host doesn't work.

Driven by this bug report: 
https://bugzilla.gnome.org/show_bug.cgi?id=779668 it seems that the 
character '@' is not treated as a valid character in the macro 
GST_ASCII_IS_STRING:

gst/gst_private.h:
#define GST_ASCII_IS_STRING(c) (g_ascii_isalnum((c)) || ((c) == '_') || \
     ((c) == '-') || ((c) == '+') || ((c) == '/') || ((c) == ':') || \
     ((c) == '.'))

I don't know if this is the reason that "user at host" is not recognized as 
a valid string, I don't know the code to be sure that this is the case, 
but it seems plausible.

Does this merit a new bug report?