feat(http1): Add http1_writev(bool) to client and server Builders

Restore a way to force queue writing strategy.

Closes #2676

src/client/client.rs

@@ -1022,6 +1022,23 @@ impl Builder {
         self
     }
 
+    /// Set whether HTTP/1 connections should try to use vectored writes,
+    /// or always flatten into a single buffer.
+    ///
+    /// Note that setting this to false may mean more copies of body data,
+    /// but may also improve performance when an IO transport doesn't
+    /// support vectored writes well, such as most TLS implementations.
+    ///
+    /// 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, enabled: bool) -> &mut Builder {
+        self.conn_builder.http1_writev(enabled);
+        self
+    }
+
     /// Set whether HTTP/1 connections will write header names as title case at
     /// the socket level.
     ///