make sure uri without host are valid

src/grammars/belle_sip_message.g

@@ -309,6 +309,8 @@ catch [ANTLR3_MISMATCHED_TOKEN_EXCEPTION]
 
 hier_part[belle_generic_uri_t* uri] returns [belle_generic_uri_t* ret=NULL]  
 :   (
+ (SLASH SLASH path_segments[NULL])=>( SLASH SLASH path_segments[uri])
+  | 
   (SLASH SLASH)=>( SLASH SLASH authority[uri] (path_segments[uri])?) 
   | 
   ( path_segments[uri]) ) 

tester/belle_generic_uri_tester.c

@@ -94,13 +94,13 @@ static void test_file_path() {
 		belle_sip_object_unref(source_uri);
 	}
 
-	//this is INVALID
-	source_uri = belle_generic_uri_parse("file:///./relative-file");
+	/*this is INVALID*/
+	source_uri = belle_generic_uri_parse("file://./relative-file");
 	CU_ASSERT_PTR_NULL(source_uri);
 
-	// instead, this should be VALID
+	/* PATH segment always start by / */
 	source_uri = belle_generic_uri_parse("./relative-file");
-	CU_ASSERT_PTR_NOT_NULL(source_uri);
+	CU_ASSERT_PTR_NULL(source_uri);
 	if (source_uri!=NULL){
 		belle_sip_object_unref(source_uri);
 	}