patch 7.4.1559

Problem:    Passing cookie to a callback is clumsy.
Solution:   Change function() to take arguments and return a partial.

src/json.c

@@ -212,6 +212,7 @@ json_encode_item(garray_T *gap, typval_T *val, int copyID, int options)
 	    break;
 
 	case VAR_FUNC:
+	case VAR_PARTIAL:
 	case VAR_JOB:
 	case VAR_CHANNEL:
 	    /* no JSON equivalent TODO: better error */