revert deactivation of expect when receiving an 100 response

lib/httpx/plugins/expect.rb

@@ -38,6 +38,23 @@ module HTTPX
 
           @headers["expect"] = "100-continue"
         end
+
+        def response=(response)
+          super
+
+          return unless @response && @response == 100 &&
+                        !@request.headers.key?("expect") &&
+                        (@state == :body || @state == :done)
+
+          # if we're past this point, this means that we just received a 100-Continue response,
+          # but the request doesn't have the expect flag, and is already flushing (or flushed) the body.
+          #
+          # this means that expect was deactivated for this request too soon, i.e. response took longer.
+          #
+          # so we have to reactivate it again.
+          request.headers["expect"] = "100-continue"
+          @response = nil
+        end
       end
 
       module ConnectionMethods