Add support for HTTP/2

[doxblek]

I ran into the situation where I had to mock an HTTP/2-only-Server.
In the application I consumed the server using reqwest. I set reqwest to HTTP/2 only by this:

let client = ClientBuilder::new().http2_prior_knowledge().build()?;

When using mockito, in contrast to the actual server, my unit test hang up (async unit test via tokio). Once I conditionally set reqwest to HTTP/2-only (using #[cfg(test)]), mockito worked as expected.

So it seams mockito is currently not supporting HTTP/2 – also I could not find any related options.

While I was able to find a workaround (that does not even has runtime overhead), it was still preferable to have that feature included.

Thank you!

[lipanski]

Fixed in https://github.com/lipanski/mockito/releases/tag/0.32.0