Keep track of added and removed headers for global test server

[bartlannoeye]

In case someone opts for the global test server instead of a per-test server, we should keep track of added and removed headers to restore to the default state after test execution.

[ivaylokenov]

The WithoutDefaultRequestHeader method should be documented here - https://github.com/ivaylokenov/MyTested.WebApi/tree/master/documentation

Also, I believe there should be a multiple headers option - WithoutDefaultRequestHeaders(params string[] keys), for example.