Consume (some) body on drop

[algesten]

Close #94

[algesten]

This is probably not possible to implement currently. If we introduce a Drop trait on Body or BodyReader, we end up in a situation that makes this code impossible:

let reader = ureq::get("http://httpbin.org/bytes/100")
    .call()?
    .body_mut()
    .with_config()
    // Reader will only read 50 bytes
    .limit(50)
    .reader();

The error is

error[E0716]: temporary value dropped while borrowed
  --> src/body/mod.rs:415:14
   |
6  |   let reader = ureq::get("https://httpbin.org/get")
   |  ______________^
7  | |     .call()?
   | |____________^ creates a temporary value which is freed while still in use
...
12 |       .reader();
   |                - temporary value is freed at the end of this statement
13 |   Ok::<_, ureq::Error>(())
14 |   } _doctest_main_src_body_mod_rs_412_0().unwrap() }
   |   - borrow might be used here, when `reader` is dropped and runs the `Drop` code for type `BodyReader`

I believe that introducing a Drop trait anywhere in the body-handling code, will trigger this error. The above code is important for ergonomics (using body_mut().xxx as opposed to intermediate let). Maybe this can be solved in future Rust.