Fix #900: Change some comments and add unit tests.

[aleksostapenko]

No description provided.

[vankoven]

I approve this PR, but an update to tempesta-test repo is required. Original issue description also described RFC violation at point 4 from Section 3.3.3, wrong status codes was sent to a client:

If this is a
       request message, the server MUST respond with a 400 (Bad Request)
       status code and then close the connection.  If this is a response
       message received by a proxy, the proxy MUST close the connection
       to the server, discard the received response, and send a 502 (Bad
       Gateway) response to the client. 

Seems like tests from long_body/test_response_wrong_length.py checks some cases of misusing Content-length headers, while others are not checked. Please, extend tests in tempesta-test repo to cover these cases.