FS-11721 refactor msrp serialize function

to be consistant with Content-Type at the end of headers if present
dynamically alloc memory to avoid buffer overflow on large message

src/include/switch_msrp.h

@@ -47,7 +47,7 @@ enum {
 	MSRP_METHOD_REPLY,
 	MSRP_METHOD_SEND,
 	MSRP_METHOD_AUTH,
-	MSRP_METHOD_REPORT,
+	MSRP_METHOD_REPORT
 };
 
 typedef enum {