add relation constructor type check

Author: aisk

lib/relation.js

@@ -19,6 +19,9 @@
    * </p>
    */
   AV.Relation = function(parent, key) {
+    if (! _.isString(key)) {
+      throw TypeError('key must be a string');
+    }
     this.parent = parent;
     this.key = key;
     this.targetClassName = null;