WebSocket ping, fixes, coverage:

* Improve test coverage
* tests for invokable in composed ops

* Update documentation
* Add License badge to README
* Target Windows 7 SDK and later
* Make role_type private
* Remove extra unused masking functions
* Allow stream reuse / reconnect after failure
* Restructure logic of composed operations
* Allow 0 for read_message_max meaning no limit
* Respect keep alive when building HTTP responses
* Check version in upgrade request
* Response with 426 status on unsupported WebSocket version
* Remove unnecessary Sec-WebSocket-Key in HTTP responses
* Rename to mask_buffer_size

* Remove maybe_throw
* Add ping, async_ping, async_on_pong
* Add ping_op
* Add pong_op
* Fix crash in accept_op
* Fix suspend in close_op
* Fix read_frame_op logic
* Fix crash in read_op
* Fix races in echo sync and async echo servers

test/http/message_v1.cpp

@@ -59,6 +59,7 @@ public:
         }
         catch(std::exception const&)
         {
+            pass();
         }
         m.headers.erase("Content-Length");
         m.headers.insert("Connection", "keep-alive");
@@ -69,7 +70,12 @@ public:
         }
         catch(std::exception const&)
         {
+            pass();
         }
+        m.version = 11;
+        m.headers.erase("Connection");
+        m.headers.insert("Connection", "close");
+        expect(! is_keep_alive(m));
     }
 
     void run() override