Concat header before method

Testing using:
node v0.8.23
jasmine-node: 1.7.1

When testing something like:
```javascript
.expectHeaderContains('Content-Type', 'json');
````
The assertion failed, but if I changed it to: 
```javascript
.expectHeaderContains('content-type', 'json');
````
it worked.

It seems the `object+"".toLowerString()` execution order happening was toLowerString() would lower case the empty string then concatenate to the object.