Cannot construct `blocking::Response` from `http::Response<blocking::Body>`

[jonhoo]

Currently, a blocking::Response can only be constructed from http::Responses with a non-blocking Body: https://github.com/seanmonstar/reqwest/blob/a0a7db0113bf3df307c4de7219fdfcf70ac0f68a/src/blocking/response.rs#L420

This was probably accidental, as (from what I can tell) other conversions in mod blocking depend on other blocking::*.

Unfortunately, changing it now would be backwards-incompatible. We also can't add a second impl From for the blocking Body type since the two implementations are potentially-overlapping and will be rejected by rustc (a type T could implement Into for both the async and blocking Body types). We should perhaps mark said impl as #[deprecated] though, with the intent of fixing the impl in the next major release.

In the meantime we could add an explicit constructor to Response along the lines of

impl Response {
    fn from_http<T: Into<super::Body>>(r: http::Response<T>) -> Self {
        // ...
    }
}

We'd also internally need a way to convert a blocking::Body into an async_impl::Body (possibly using blocking::Body::into_async, I'm not sure).