forEach() requires 'this' argument

Otherwise, this is evaluated to undefined, which causes cryptic errors like:

`Cannot read property 'method' of undefined`

instead of:

`writeFileSync received wrong arguments(...)`

lib/sinon/mock-expectation.js

@@ -182,7 +182,7 @@ var mockExpectation = {
                 mockExpectation.fail(this.method + " received wrong arguments " + format(args) +
                     ", expected " + format(expectedArguments));
             }
-        });
+        }, this);
     },
 
     allowsCall: function allowsCall(thisValue, args) {