`put` returns the number of bytes actually transferred
from the input buffers.
Actions Required:
* Return the number of bytes actually transferred from the
input buffers in user defined `Body::writer::put` functions.
The is_deferred nested type is removed from the BodyReader
requirements.
Performance for sending messages with `file_body` was cut
almost in half with is_deferred as `std::true_type` since
it caused double the number of socket writes. If the
deferred behavior is absolutely necessary, callers can get
the same effect by manually sending the headers first.
Actions Required:
* Callers who need the behavior of is_deferred as `std::true_type`
should manually construct a serializer and serialize the header
first, followed by the body.
