Previously, using `.send()` on `Request` would require to set either the
Transfer-Encoding or the Content-Length header.
In an effort to provide better ergonomics for this library and to avoid
making users fall in a not-so obvious pitfall, the library should build a
valid Request without asking the user to mess around with the headers.
This commit attempts to fix this issue: if the user use `.send()` to
provide an unknown sized reader, the chunked Transfer-Encoding will be
used. Of course, there are prior checks to ensure we do not override the
user wish, like if the user already set a Content-Length or a
Transfer-Encoding.
This commit fix an edge case where the Content-Length would not be
automatically set if the Transfer-Encoding is set but not chunked.
Deluvi