Add support for connections as body in POST and friends

[s-u]

Allows body to be a connection. It uses readfunction (just like form_file) and chunked transfer encoding to allow POSTs of non-seekable content where length is not known a priori (e.g. pipe() or others). It is implemented in body_config so works really for any method that requires body.

[hadley]

httr has been superseded in favour of httr2, so is no longer under active development. Thanks for using httr, thanks for contributing, and my apologies that your contribution never made it into the package.