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/request.rs

@@ -210,8 +210,10 @@ impl Request {
 
     /// Send data from a reader.
     ///
-    /// This uses [chunked transfer encoding](https://tools.ietf.org/html/rfc7230#section-4.1).
-    /// The caller is responsible for setting the Transfer-Encoding: chunked header.
+    /// If no Content-Length and Transfer-Encoding header has been set, it uses the [chunked transfer encoding](https://tools.ietf.org/html/rfc7230#section-4.1).
+    ///
+    /// The caller may set the Content-Length header to the expected byte size of the reader if is
+    /// known.
     ///
     /// The input from the reader is buffered into chunks of size 16,384, the max size of a TLS fragment.
     ///
@@ -222,7 +224,6 @@ impl Request {
     ///
     /// let resp = ureq::post("http://localhost/example-upload")
     ///     .set("Content-Type", "text/plain")
-    ///     .set("Transfer-Encoding", "chunked")
     ///     .send(read);
     /// ```
     pub fn send(&mut self, reader: impl Read + 'static) -> Response {