fix($http): ensure case-insens. header overriding

If user send content-type header, both content-type and default
Content-Type headers were sent. Now default header overriding is
case-insensitive.

As per [RFC 2616](http://www.ietf.org/rfc/rfc2616.txt), "[header] Field 
names are case-insensitive".

Before this fix, the following test would fail:

``` javascript
it('should override default headers with custom in a case insensitive manner', function() {
  $httpBackend.expect('POST', '/url', 'messageBody', function(headers) {
    return headers['accept'] == 'Rewritten' &&
           headers['content-type'] == 'Rewritten' &&
           headers['Accept'] === undefined &&
           headers['Content-Type'] === undefined;
  }).respond('');

  $http({url: '/url', method: 'POST', data: 'messageBody', headers: {
    'accept': 'Rewritten',
    'content-type': 'Rewritten'
  }});
  $httpBackend.flush();
});
```