* HTTP.{cpp,h} : add add_header() variant with std::string

HTTP.cpp

@@ -193,6 +193,10 @@ namespace http {
     return out;
   }
 
+  void HTTPMsg::add_header(const char *name, std::string & value, bool replace) {
+    add_header(name, value.c_str(), replace);
+  }
+
   void HTTPMsg::add_header(const char *name, const char *value, bool replace) {
     std::size_t count = headers.count(name);
     if (count && !replace)

HTTP.h

@@ -57,6 +57,7 @@ namespace http {
   struct HTTPMsg {
     std::map<std::string, std::string> headers;
 
+    void add_header(const char *name, std::string & value, bool replace = false);
     void add_header(const char *name, const char *value, bool replace = false);
     void del_header(const char *name);