feat(client): allow HTTP/0.9 responses behind a flag (#2473)

Fixes #2468

src/proto/h1/conn.rs

@@ -47,6 +47,7 @@ where
                 #[cfg(feature = "ffi")]
                 preserve_header_case: false,
                 title_case_headers: false,
+                h09_responses: false,
                 notify_read: false,
                 reading: Reading::Init,
                 writing: Writing::Init,
@@ -78,6 +79,11 @@ where
         self.state.title_case_headers = true;
     }
 
+    #[cfg(feature = "client")]
+    pub(crate) fn set_h09_responses(&mut self) {
+        self.state.h09_responses = true;
+    }
+
     #[cfg(feature = "server")]
     pub(crate) fn set_allow_half_close(&mut self) {
         self.state.allow_half_close = true;
@@ -146,6 +152,7 @@ where
                 req_method: &mut self.state.method,
                 #[cfg(feature = "ffi")]
                 preserve_header_case: self.state.preserve_header_case,
+                h09_responses: self.state.h09_responses,
             }
         )) {
             Ok(msg) => msg,
@@ -157,6 +164,9 @@ where
 
         debug!("incoming body is {}", msg.decode);
 
+        // Prevent accepting HTTP/0.9 responses after the initial one, if any.
+        self.state.h09_responses = false;
+
         self.state.busy();
         self.state.keep_alive &= msg.keep_alive;
         self.state.version = msg.head.version;
@@ -753,6 +763,7 @@ struct State {
     #[cfg(feature = "ffi")]
     preserve_header_case: bool,
     title_case_headers: bool,
+    h09_responses: bool,
     /// Set to true when the Dispatcher should poll read operations
     /// again. See the `maybe_notify` method for more.
     notify_read: bool,