fix; proper support for .update(doc) (Fix #3221)

Author: vkarpov15

lib/query.js

@@ -1989,20 +1989,27 @@ Query.prototype._execUpdate = function(castedQuery, castedDoc, options, callback
 
 Query.prototype.update = function(conditions, doc, options, callback) {
   if ('function' === typeof options) {
-    // Scenario: update(conditions, doc, callback)
+    // .update(conditions, doc, callback)
     callback = options;
     options = null;
   } else if ('function' === typeof doc) {
-    // Scenario: update(doc, callback);
+    // .update(doc, callback);
     callback = doc;
     doc = conditions;
     conditions = {};
     options = null;
   } else if ('function' === typeof conditions) {
+    // .update(callback)
     callback = conditions;
     conditions = undefined;
     doc = undefined;
     options = undefined;
+  } else if (typeof conditions === 'object' && !doc && !options && !callback) {
+    // .update(doc)
+    doc = conditions;
+    conditions = undefined;
+    options = undefined;
+    callback = undefined;
   }
 
   // make sure we don't send in the whole Document to merge()