fix: support both lowercase and camelcase headers' keys

Solved a problem with the letter case of headers' keys. 

When I used httplib2 python library I noticed that Cocaine couldn't receive multipart body and my Flask application hadn't any uploaded files in request.files object. 

The cause of this problem was different letter case in headers and in tornado.wsgi.WSGIContainer.environ method which can process only camelcase headers. 

```python
if "Content-Type" in request.headers:
            environ["CONTENT_TYPE"] = request.headers.pop("Content-Type")
```

I fixed the problem by using HTTPHeaders decorator object that extends built-in dict class. It maintains ``Http-Header-Case`` for all keys.