Set chunked Transfer-Encoding when using send

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.

src/body.rs

@@ -66,7 +66,7 @@ impl SizedReader {
 impl Payload {
     pub fn into_read(self) -> SizedReader {
         match self {
-            Payload::Empty => SizedReader::new(None, Box::new(empty())),
+            Payload::Empty => SizedReader::new(Some(0), Box::new(empty())),
             Payload::Text(text, _charset) => {
                 #[cfg(feature = "charset")]
                 let bytes = {