Fix VS2019 warning due to discarding nodiscard value

Normally I'd just delete this since we explicitly copy it into the
lambda anyway, but there are a lot of comments here about that this may
be needed(?)

warning C4834: discarding return value of function with 'nodiscard'
attribute

Author: robinlinden

autobahn/wamp_session.ipp

@@ -790,7 +790,7 @@ inline void wamp_session::process_challenge(wamp_message&& message)
             });
 
             // make sure the context_response is copied into this lambda...
-            context_response.get();
+            (void)context_response.get();
         } catch (const std::exception&) {
             if (m_debug_enabled) {
                 std::cerr << "failed to handle authentication" << std::endl;