`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.
* FieldsReader now requires chunked() and keep_alive()
* serializer logic calls into the reader to calculate
the message metadata
* Removed some of the previous requirements of FieldsReader
Actions Required:
* Implement chunked() and keep_alive() for user defined FieldsReader types.
Vinnie Falco