Fix broken build and tests with --no-default-features

[jirutka]

No description provided.

[iddm]

I sincerely apologise for the late response. Many things are going on right now. I will have time to review either this week or the next week, unfortunately.

[iddm]

Hi again! First of all, thank you for taking time to contribute! I have a little proposal for this pull request. As it already touches what I have wanted to change for quite some time now, perhaps, now it is a good time to do that, of course, if you don't mind. The thing is, the HeaderMap used in request.rs isn't really needed there in the structure called Request. What we need there is just something what may hold unique associations of strings. A good candidate, for example, would be the std::collections::HashMap. Can you please remove the http crate dependency for good and replace the code using it so that it uses just a hash map, and in case of the client feature being enabled, it would be creating a http::HeaderMap object on-the-fly? If you don't have time for this, I can just merge the code right now and do this myself, no problem at all.

In case you were to ask me why I wanted to do this, that is because I want the crate to have as minimal explicit dependencies as possible, so that anyone can use it anywhere as having it as a dependency (hopefully) wouldn't ever cause a dependency hell problem. This is also why the client feature is not enabled by default and is optional for the crate to work.