Add asio_handler_invoke overloads for stream algorithms:

fix #1012

This fixes a bug where asynchronous stream alogrithms do not
work correctly with the legacy `io_service::strand` implementation.

test/beast/http/write.cpp

@@ -22,6 +22,8 @@
 #include <boost/beast/test/yield_to.hpp>
 #include <boost/beast/unit_test/suite.hpp>
 #include <boost/asio/error.hpp>
+#include <boost/asio/io_service.hpp>
+#include <boost/asio/strand.hpp>
 #include <sstream>
 #include <string>
 
@@ -857,6 +859,49 @@ public:
         ioc.run();
     }
 
+    struct copyable_handler
+    {
+        template<class... Args>
+        void
+        operator()(Args&&...) const
+        {
+        }
+    };
+
+    void
+    testAsioHandlerInvoke()
+    {
+        // make sure things compile, also can set a
+        // breakpoint in asio_handler_invoke to make sure
+        // it is instantiated.
+        {
+            boost::asio::io_context ioc;
+            boost::asio::io_service::strand s{ioc};
+            test::stream ts{ioc};
+            flat_buffer b;
+            request<empty_body> m;
+            request_serializer<empty_body, fields> sr{m};
+            async_write_some(ts, sr, s.wrap(copyable_handler{}));
+        }
+        {
+            boost::asio::io_context ioc;
+            boost::asio::io_service::strand s{ioc};
+            test::stream ts{ioc};
+            flat_buffer b;
+            request<empty_body> m;
+            request_serializer<empty_body, fields> sr{m};
+            async_write(ts, sr, s.wrap(copyable_handler{}));
+        }
+        {
+            boost::asio::io_context ioc;
+            boost::asio::io_service::strand s{ioc};
+            test::stream ts{ioc};
+            flat_buffer b;
+            request<empty_body> m;
+            async_write(ts, m, s.wrap(copyable_handler{}));
+        }
+    }
+
     void
     run() override
     {
@@ -878,6 +923,7 @@ public:
                 testWriteStream<test_body< true, false>>(yield);
                 testWriteStream<test_body< true,  true>>(yield);
             });
+        testAsioHandlerInvoke();
     }
 };