Filter 'x-content-type-options' from twitter's response headers

Twitter responds with `x-content-type-options` set to `nosniff` (rightfully so), unfortunately this leads to strict MIME errors when used in browsers (like we do with the twitterwall), so that the script is not executed:

![reject_js_2014](https://cloud.githubusercontent.com/assets/82050/4206140/ca0b022c-3847-11e4-8ab4-05e214797b75.png)

This just filters this out from the response headers.