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.

doc/quickref.xml

@@ -107,6 +107,7 @@
           <bridgehead renderas="sect3">Functions</bridgehead>
           <simplelist type="vert" columns="1">
             <member><link linkend="beast.ref.websocket__async_teardown">async_teardown</link></member>
+            <member><link linkend="beast.ref.websocket__is_upgrade">is_upgrade</link></member>
             <member><link linkend="beast.ref.websocket__teardown">teardown</link></member>
           </simplelist>
           <bridgehead renderas="sect3">Options</bridgehead>