fix #438

问题：edge模式，推流时异常断开。
    解决方法：增加edge模式推流检测。

trunk/src/app/srs_app_source.cpp

@@ -1355,6 +1355,11 @@ bool SrsSource::can_publish()
     return _can_publish;
 }
 
+bool SrsSource::proxy_can_publish()
+{
+    return publish_edge->can_publish();
+}
+
 int SrsSource::on_meta_data(SrsCommonMessage* msg, SrsOnMetaDataPacket* metadata)
 {
     int ret = ERROR_SUCCESS;