Bugfix: Allow multiple headers with the same name

Sometimes multiple headers with the same name/key are added by the legacy application. This bugfix makes sure these headers are added correctly to symfony2.

Example: Set-Cookie

The lagacy application can set more than one cookie with one request. 

Before the bugfix, only one "Set-Cookie" would be send to the client.
Merge pull request #6 from stijink/master

Bugfix: Allow multiple headers with the same name