Enable aborting requests by returning `:abort` from on_body callback.

libcurl allows you to pass set `writefunction` which usually writes the
body. It is passed chunkes of data. Usually the number of bytes taken
care of is returned.

> If that amount differs from the amount passed  to your  function,
> it'll signal an error to the library.  This will abort the transfer
> and return CURLE_WRITE_ERROR.

Ethon now allows returning `:abort` from `on_body` callbacks which in
turn will abort the request.