ext/soup/gstsouphttpsrc.c: Let the proxy property default to the content of the $http_proxy environment variable.

Original commit message from CVS:
Patch by: Wouter Cloetens <wouter at mind dot be>
* ext/soup/gstsouphttpsrc.c: (gst_souphttp_src_init):
Let the proxy property default to the content of the $http_proxy
environment variable.