feat(client): add http1_read_buf_exact_size Builder option

This changes the read buffer strategy from being adaptive to always
using an exact size for the buffer.

src/proto/h1/conn.rs

@@ -63,6 +63,10 @@ where I: AsyncRead + AsyncWrite,
         self.io.set_max_buf_size(max);
     }
 
+    pub fn set_read_buf_exact_size(&mut self, sz: usize) {
+        self.io.set_read_buf_exact_size(sz);
+    }
+
     pub fn set_write_strategy_flatten(&mut self) {
         self.io.set_write_strategy_flatten();
     }