resolves #48: custom css ineffectual behind reverse proxy

Caddy (and probably other webservers) add a Content-Type header
when one was not supplied by the application backend. Under Chromium,
the css file is not interpreted if the webserver delivers it with
Content-Type: text/plain, althugh it does work if no Content-Type
header is set. To prevent the frontend proxy from adding the wrong
header, set it correctly in the first place

src/server.cpp

@@ -217,6 +217,7 @@ public:
                 }
             } else if(resource.compare("/custom/style.css") == 0) {
                 c->set_status(websocketpp::http::status_code::ok);
+                c->append_header("Content-Type", "text/css; charset=utf-8");
                 c->append_header("Content-Transfer-Encoding", "binary");
                 c->set_body(laminar.getCustomCss());
             } else if(resources.handleRequest(resource, &start, &end, &content_type)) {