Add support for supplying content length in requests

[szakharchenko]

This enables support for servers which do not support use of chunked transfer encoding in requests, e.g. emulated devices.

[szakharchenko]

See also https://github.com/timonreinhard/wemo-client/issues/63.

[jimmywarting]

Hmm, I think ideally we should set content-length header whenever the size is known... shouldn't need a option for this...