Add integration tests for HTTP Server

[tomaszmichalak]

Implement scenarios defined here: https://github.com/Knotx/knotx-server-http/tree/master/it-test/src/test/java/io/knotx/server

[malaskowski]

Some of the tests introduced in the #15

[malaskowski]

This should include the test that was removed in Knotx/knotx-stack#60
The test checks if the server properly computes content-length and content-type headers.