Fixed warnings: variable 'Req_' exported from 'case'

Fixed 2 warnings:
```
cowboy/src/cowboy_req.erl:625: Warning: variable 'Req1' exported from 'case' (line 617)
cowboy/src/cowboy_req.erl:946: Warning: variable 'Req2' exported from 'case' (line 915)
```