Avoid searches to permanent overwrite options

lib/bing.js

@@ -50,11 +50,10 @@ var Bing = function( options ) {
       throw "Error: Callback function required!";
     }
 
-    var opts = this.options;
+    // Create a copy of the options, to avoid permanent overwrites
+    var opts = JSON.parse(JSON.stringify(this.options));
 
-    if (options !== null) {
-      opts = _.extend(this.options, options);
-    }
+    _.extend(opts, options);
 
     var reqUri = opts.rootUri
                    + vertical