Fix a type error that would not compile on Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)

Github-Pull: #8513
Rebased-From: 8194a6e525

src/httpserver.h

@@ -35,7 +35,7 @@ void InterruptHTTPServer();
 void StopHTTPServer();
 
 /** Handler for requests to a certain HTTP path */
-typedef boost::function<void(HTTPRequest* req, const std::string &)> HTTPRequestHandler;
+typedef boost::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.