Include redirected items when parsing headers.

Undocumented Apache [annoyance](http://goo.gl/tRKHba). In some cases when using mod_rewrite *REDIRECT_* prefix is added to the headers.  This happens for example when using workaround to make HTTP Basic Authentication work together with FastCGI.

```RewriteRule .* - [env=HTTP_AUTHORIZATION:%{HTTP:Authorization}]```

Currently Slim does not include these items when parsing the headers. Only way to access them is through *$_SERVER* superglobal.