Ensure _.keys and _.keysIn always skip the constructor property on prototype objects.

lodash.js

@@ -6572,8 +6572,11 @@
      * // => ['x', 'y'] (property order is not guaranteed across environments)
      */
     var keys = !nativeKeys ? shimKeys : function(object) {
-      var length = object ? object.length : 0;
-      if (typeof length == 'number' && length > 0) {
+      var ctor = object && object.constructor,
+          length = object ? object.length : 0;
+
+      if (typeof length == 'number' && length > 0 ||
+          (ctor && object === ctor.prototype)) {
         return shimKeys(object);
       }
       return isObject(object) ? nativeKeys(object) : [];
@@ -6609,7 +6612,9 @@
           (support.nonEnumArgs && isArguments(object))) && length) >>> 0;
 
       var keyIndex,
+          ctor = object.constructor,
           index = -1,
+          isProto = ctor && object === ctor.prototype,
           maxIndex = length - 1,
           result = Array(length),
           skipIndexes = length > 0,
@@ -6620,7 +6625,8 @@
         result[index] = String(index);
       }
       for (var key in object) {
-        if (!(skipProto && key == 'prototype') &&
+        if (!(isProto && key == 'constructor') &&
+            !(skipProto && key == 'prototype') &&
             !(skipErrorProps && (key == 'message' || key == 'name')) &&
             !(skipIndexes && (keyIndex = +key, keyIndex > -1 && keyIndex <= maxIndex && keyIndex % 1 == 0))) {
           result.push(key);
@@ -6631,11 +6637,10 @@
       // attribute of an existing property and the `constructor` property of a
       // prototype defaults to non-enumerable.
       if (support.nonEnumShadows && object !== objectProto) {
-        var ctor = object.constructor;
         index = -1;
         length = shadowedProps.length;
 
-        if (object === (ctor && ctor.prototype)) {
+        if (isProto) {
           var className = object === stringProto ? stringClass : object === errorProto ? errorClass : toString.call(object),
               nonEnum = nonEnumProps[className];
         }