Fix: clear reponse text before doing curl request

Response text is appended to whatever is in _text, so that should be
cleared before every request.

src/requester.cpp

@@ -74,6 +74,7 @@ std::string requester::post(const std::string& url, const std::string& json) {
     curl_easy_setopt(_h.get(), CURLOPT_HTTPHEADER, headers);
     curl_easy_setopt(_h.get(), CURLOPT_WRITEDATA, this);
 
+    _text = "";
     auto success = curl_easy_perform(_h.get());
     if (success != CURLE_OK) {
         std::ostringstream oss;