add JSONValue.isNull

- short-access similar to array, str, object...
- val.type == JSON_TYPE.NULL is fairly verbose

std/json.d

@@ -197,6 +197,12 @@ struct JSONValue
         return store.array;
     }
 
+    /// Test whether the type is $(D JSON_TYPE.NULL)
+    @property bool isNull() const
+    {
+        return type == JSON_TYPE.NULL;
+    }
+
     private void assign(T)(T arg)
     {
         static if(is(T : typeof(null)))
@@ -342,7 +348,7 @@ struct JSONValue
         return *enforce!JSONException(k in store.object,
                                         "Key not found: " ~ k);
     }
-    
+
     /// Operator sets $(D value) for element of JSON object by $(D key)
     /// If JSON value is null, then operator initializes it with object and then
     /// sets $(D value) for it.
@@ -352,10 +358,10 @@ struct JSONValue
     {
         enforceEx!JSONException(type == JSON_TYPE.OBJECT || type == JSON_TYPE.NULL,
                                 "JSONValue must be object or null");
-        
+
         if(type == JSON_TYPE.NULL)
             this = (JSONValue[string]).init;
-        
+
         store.object[key] = value;
     }
 
@@ -999,6 +1005,12 @@ unittest
         assert(index == (value.integer-3));
     }
 
+    jv = null;
+    assert(jv.type == JSON_TYPE.NULL);
+    assert(jv.isNull);
+    jv = "foo";
+    assert(!jv.isNull);
+
     jv = JSONValue("value");
     assert(jv.type == JSON_TYPE.STRING);
     assert(jv.str == "value");
@@ -1203,28 +1215,28 @@ deprecated unittest
 unittest
 {
     // Bugzilla 12969
-    
+
     JSONValue jv;
     jv["int"] = 123;
-    
+
     assert(jv.type == JSON_TYPE.OBJECT);
     assert("int" in jv);
     assert(jv["int"].integer == 123);
-    
+
     jv["array"] = [1, 2, 3, 4, 5];
-    
+
     assert(jv["array"].type == JSON_TYPE.ARRAY);
     assert(jv["array"][2].integer == 3);
-    
+
     jv["str"] = "D language";
     assert(jv["str"].type == JSON_TYPE.STRING);
     assert(jv["str"].str == "D language");
-    
+
     jv["bool"] = false;
     assert(jv["bool"].type == JSON_TYPE.FALSE);
-    
+
     assert(jv.object.length == 4);
-    
+
     jv = [5, 4, 3, 2, 1];
     assert( jv.type == JSON_TYPE.ARRAY );
     assert( jv[3].integer == 2 );