feat(lib): Setting http1_writev(true) will now force writev queue usage

Previously, calling `http1_writev(true)` would just keep the default behavior, which was to auto detect if writev was optimal. Now, the auto-detection is still default, but explicitly calling `http1_writev(true)` will skip the auto-detection, and always use writev queue strategy.

Closes #2282

src/client/mod.rs

@@ -964,7 +964,11 @@ impl Builder {
     /// but may also improve performance when an IO transport doesn't
     /// support vectored writes well, such as most TLS implementations.
     ///
-    /// Default is `true`.
+    /// Setting this to true will force hyper to use queued strategy
+    /// which may eliminate unnecessary cloning on some TLS backends
+    ///
+    /// Default is `auto`. In this mode hyper will try to guess which
+    /// mode to use
     pub fn http1_writev(&mut self, val: bool) -> &mut Self {
         self.conn_builder.h1_writev(val);
         self