Add is_upgrade() free function:

fix #195

This function returns `true` when the passed HTTP Request
indicates a WebSocket Upgrade. It does not validate the
contents of the fields: it just trivially accepts requests
which can only be a WebSocket Upgrade message.

Callers who wish to manually read HTTP requests in their
server implementation can use this function to determine if
the request should be routed to an instance of websocket::stream.

include/beast/websocket/impl/rfc6455.ipp

@@ -0,0 +1,36 @@
+//
+// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BEAST_WEBSOCKET_IMPL_RFC6455_IPP
+#define BEAST_WEBSOCKET_IMPL_RFC6455_IPP
+
+#include <beast/http/rfc7230.hpp>
+
+namespace beast {
+namespace websocket {
+
+template<class Fields>
+bool
+is_upgrade(http::header<true, Fields> const& req)
+{
+    if(req.version < 11)
+        return false;
+    if(req.method != "GET")
+        return false;
+    if(! http::is_upgrade(req))
+        return false;
+    if(! http::token_list{req.fields["Upgrade"]}.exists("websocket"))
+        return false;
+    if(! req.fields.exists("Sec-WebSocket-Version"))
+        return false;
+    return true;
+}
+
+} // websocket
+} // beast
+
+#endif

include/beast/websocket/rfc6455.hpp

@@ -10,13 +10,34 @@
 
 #include <beast/config.hpp>
 #include <beast/core/static_string.hpp>
-#include <boost/optional.hpp>
+#include <beast/http/message.hpp>
 #include <array>
 #include <cstdint>
 
 namespace beast {
 namespace websocket {
 
+/** Returns `true` if the specified HTTP request is a WebSocket Upgrade.
+
+    This function returns `true` when the passed HTTP Request
+    indicates a WebSocket Upgrade. It does not validate the
+    contents of the fields: it just trivially accepts requests
+    which could only possibly be a valid or invalid WebSocket
+    Upgrade message.
+
+    Callers who wish to manually read HTTP requests in their
+    server implementation can use this function to determine if
+    the request should be routed to an instance of
+    @ref websocket::stream.
+
+    @param req The HTTP Request object to check.
+
+    @return `true` if the request is a WebSocket Upgrade.
+*/
+template<class Fields>
+bool
+is_upgrade(beast::http::header<true, Fields> const& req);
+
 /** WebSocket frame header opcodes. */
 enum class opcode : std::uint8_t
 {
@@ -186,4 +207,6 @@ struct close_reason
 } // websocket
 } // beast
 
+#include <beast/websocket/impl/rfc6455.ipp>
+
 #endif