set default extension offer string

Currently the offer is for default settings and advertises the ability
to accept any counteroffer.

websocketpp/extensions/permessage_deflate/disabled.hpp

@@ -88,6 +88,17 @@ public:
         return false;
     }
 
+    /// Generate extension offer
+    /**
+     * Creates an offer string to include in the Sec-WebSocket-Extensions
+     * header of outgoing client requests.
+     *
+     * @return A WebSocket extension offer string for this extension
+     */
+    std::string generate_offer() const {
+        return "";
+    }
+
     /// Compress bytes
     /**
      * @param [in] in String to compress

websocketpp/extensions/permessage_deflate/enabled.hpp

@@ -439,7 +439,8 @@ public:
      * @return A WebSocket extension offer string for this extension
      */
     std::string generate_offer() const {
-        return "";
+        // TODO: this should be dynamically generated based on user settings
+        return "permessage-deflate; client_no_context_takeover; client_max_window_bits";
     }
 
     /// Validate extension response