Allow to avoid a string copy in string_response constructor (etr#201)

Author: fechantillac-antidot

src/httpserver/string_response.hpp

@@ -36,12 +36,12 @@ class string_response : public http_response
         string_response() = default;
 
         explicit string_response(
-                const std::string& content,
+                std::string content,
                 int response_code = http::http_utils::http_ok,
                 const std::string& content_type = http::http_utils::text_plain
         ):
             http_response(response_code, content_type),
-            content(content)
+            content(std::move(content))
         {
         }