Disallow mixed modifier/non-modifier fields in updates

We expect modifiers to be either modifiers or replacements, not a
mix. MongoDB does this check also, but do it in meteor too as a safety
belt.

packages/mongo/mongo_driver.js

@@ -514,10 +514,20 @@ MongoConnection.prototype._update = function (collection_name, selector, mod,
 };
 
 var isModificationMod = function (mod) {
-  for (var k in mod)
-    if (k.substr(0, 1) === '$')
-      return true;
-  return false;
+  var isReplace = false;
+  var isModify = false;
+  for (var k in mod) {
+    if (k.substr(0, 1) === '$') {
+      isModify = true;
+    } else {
+      isReplace = true;
+    }
+  }
+  if (isModify && isReplace) {
+    throw new Error(
+      "Update parameter cannot have both modifier and non-modifier fields.");
+  }
+  return isModify;
 };
 
 var NUM_OPTIMISTIC_TRIES = 3;

packages/mongo/mongo_livedata_tests.js

@@ -3068,7 +3068,7 @@ Meteor.isServer && testAsyncMulti("mongo-livedata - update with replace forbidde
 
     test.throws(function () {
       c.update(id, { foo3: "bar3", $set: { blah: 1 } });
-    }, "cannot be mixed");
+    }, "cannot have both modifier and non-modifier fields");
     test.equal(c.findOne(id), { _id: id, foo2: "bar2" });
   }
 ]);