Build a blocking body from bytes::Bytes to allow copy-free chained network calls.

[varoonp123]

Currently, if I want to use the reqwest::blocking::Client, the response can give me a bytes::Bytes but afaik there is no easy/cheap way to turn that into an owned contiguous bytes type needed to build a blocking body. As of 0.10.10, a blocking body can be constructed from a file, &'static [u8], String, &'static str, and Vec.

The async API DOES allow a body to be built from Bytes which is extremely convenient for getting bytes from one url and putting them at another url. My application would benefit greatly from minimizing copying bytes in these get-put chains and this trait implementation seems to be a natural way to address what I imagine is not an extremely uncommon use case. This trait implementation in the blocking API IMO makes sense with the design philosophy of the library, makes the blocking API more consistent with the async one, and allows greater composability within and between the clients.

While this seems like the simplest/minimal change that will effectively address this pattern of guiding bytes from a get response to a put request, other alternatives to this trait implementation include:

1. Having a more direct blocking return type (blocking::Response.vec() or something) (I dont want to use std::String as an intermediate step)
2. impl<T: AsRef<[u8]>> From<T> for blocking::Body
3. Some other way that I dont know of to transform bytes::Bytes into a Vec<u8> without copying the bytes?
4. Just clone the bytes

[seanmonstar]

Sounds good, I think this was just an oversight.