Fix compile error "chosen constructor is explicit in copy-initialization"

src/asio_server_response_impl.h

@@ -46,7 +46,7 @@ enum class response_state {
 class response_impl {
 public:
   response_impl();
-  void write_head(unsigned int status_code, header_map h = {});
+  void write_head(unsigned int status_code, header_map h = header_map{});
   void end(std::string data = "");
   void end(generator_cb cb);
   void write_trailer(header_map h);
@@ -56,7 +56,7 @@ public:
   void cancel(uint32_t error_code);
 
   response *push(boost::system::error_code &ec, std::string method,
-                 std::string raw_path_query, header_map h = {}) const;
+                 std::string raw_path_query, header_map) const;
 
   boost::asio::io_service &io_service();
 

src/includes/nghttp2/asio_http2_client.h

@@ -156,7 +156,7 @@ public:
   // succeeds, or nullptr and |ec| contains error message.
   const request *submit(boost::system::error_code &ec,
                         const std::string &method, const std::string &uri,
-                        header_map h = {}) const;
+                        header_map h = header_map{}) const;
 
   // Submits request to server using |method| (e.g., "GET"), |uri|
   // (e.g., "http://localhost/") and optionally additional header
@@ -165,7 +165,7 @@ public:
   // contains error message.
   const request *submit(boost::system::error_code &ec,
                         const std::string &method, const std::string &uri,
-                        std::string data, header_map h = {}) const;
+                        std::string data, header_map h = header_map{}) const;
 
   // Submits request to server using |method| (e.g., "GET"), |uri|
   // (e.g., "http://localhost/") and optionally additional header
@@ -174,7 +174,7 @@ public:
   // nullptr and |ec| contains error message.
   const request *submit(boost::system::error_code &ec,
                         const std::string &method, const std::string &uri,
-                        generator_cb cb, header_map h = {}) const;
+                        generator_cb cb, header_map h = header_map{}) const;
 
 private:
   std::unique_ptr<session_impl> impl_;

src/includes/nghttp2/asio_http2_server.h

@@ -71,7 +71,7 @@ public:
 
   // Write response header using |status_code| (e.g., 200) and
   // additional header fields in |h|.
-  void write_head(unsigned int status_code, header_map h = {}) const;
+  void write_head(unsigned int status_code, header_map h = header_map{}) const;
 
   // Sends |data| as request body.  No further call of end() is
   // allowed.
@@ -103,7 +103,8 @@ public:
   // nullptr and error code is filled in |ec|.  Be aware that the
   // header field name given in |h| must be lower-cased.
   const response *push(boost::system::error_code &ec, std::string method,
-                       std::string raw_path_query, header_map h = {}) const;
+                       std::string raw_path_query,
+                       header_map h = header_map{}) const;
 
   // Returns status code.
   unsigned int status_code() const;