Fix a type error that would not compile on Apple LLVM version 6.1.0...

src/httpserver.h

@@ -33,7 +33,7 @@ void InterruptHTTPServer();
 void StopHTTPServer();
 
 /** Handler for requests to a certain HTTP path */
-typedef std::function<void(HTTPRequest* req, const std::string &)> HTTPRequestHandler;
+typedef std::function<bool(HTTPRequest* req, const std::string &)> HTTPRequestHandler;
 /** Register handler for prefix.
  * If multiple handlers match a prefix, the first-registered one will
  * be invoked.