Fix for incorrect Content-Length: 0 handling

If the Content-Length header is set to 0, the client must not read the content from the stream, otherwise it hangs waiting for data. 
Redirect responses (302, 307) usally have no content and have Content-Length: 0

src/request.rs

@@ -510,10 +510,10 @@ impl<'a> RequestBuilder<'a> {
             let deadline = Instant::now() + timeout;
             copy_with_timeout(stream, writer, deadline)?;
         } else {
-            let num_bytes = res.content_len().unwrap_or(0);
-
-            if num_bytes > 0 {
-                copy_exact(stream, writer, num_bytes - body_part.len())?;
+            if let Some(num_bytes) = res.content_len() {
+                if num_bytes > 0 {
+                    copy_exact(stream, writer, num_bytes - body_part.len())?;
+                }
             } else {
                 io::copy(stream, writer)?;
             }