feat(client): add conn::Builder::max_buf_size()

This allows users to configure a limit to client connections' read and
write buffers.

Closes hyperium#1748

src/client/conn.rs

@@ -75,6 +75,7 @@ pub struct Builder {
     h1_writev: bool,
     h1_title_case_headers: bool,
     h1_read_buf_exact_size: Option<usize>,
+    max_buf_size: Option<usize>,
     http2: bool,
 }
 
@@ -435,6 +436,7 @@ impl Builder {
             h1_writev: true,
             h1_read_buf_exact_size: None,
             h1_title_case_headers: false,
+            max_buf_size: None,
             http2: false,
         }
     }
@@ -462,6 +464,24 @@ impl Builder {
         self.h1_read_buf_exact_size = sz;
         self
     }
+
+    /// Set the maximum buffer size for the connection.
+    ///
+    /// Default is ~400kb.
+    ///
+    /// # Panics
+    ///
+    /// The minimum value allowed is 8192. This method panics if the passed `max` is less than the minimum.
+    pub fn max_buf_size(&mut self, max: usize) -> &mut Self {
+        assert!(
+            max >= proto::h1::MINIMUM_MAX_BUFFER_SIZE,
+            "the max_buf_size cannot be smaller than the minimum that h1 specifies."
+        );
+
+        self.max_buf_size = Some(max);
+        self
+    }
+
     /// Sets whether HTTP2 is required.
     ///
     /// Default is false.
@@ -510,6 +530,9 @@ where
             if let Some(sz) = self.builder.h1_read_buf_exact_size {
                 conn.set_read_buf_exact_size(sz);
             }
+            if let Some(max) = self.builder.max_buf_size {
+                conn.set_max_buf_size(max);
+            }
             let cd = proto::h1::dispatch::Client::new(rx);
             let dispatch = proto::h1::Dispatcher::new(cd, conn);
             Either::A(dispatch)