MXS-1220: Expand HttpResponse class

The class now generates default headers. The ETag and Last-Modified tags
do not represent any actual modification time or resource hash.

The basic functionality of the HTTP responses is tested by the core test
suite. More advanced testing of the whole REST API is still required.

Removed the static `create` functions as only the JSON parsing version
could generated errors and even then the errors were unlikely. By
replacing the static creator function with a normal constructor, the
HttpResponse class can now also be created on the stack making its use
easier.

server/core/adminclient.cc

@@ -13,6 +13,7 @@
 
 #include "maxscale/adminclient.hh"
 #include "maxscale/httprequest.hh"
+#include "maxscale/httpresponse.hh"
 
 #include <string>
 #include <sstream>
@@ -77,14 +78,9 @@ static bool read_request(int fd, string& output)
     return true;
 }
 
-static bool write_response(int fd, enum http_code code, const string& body)
+static bool write_response(int fd, const string& body)
 {
-    string payload = "HTTP/1.1 ";
-    payload += http_code_to_string(code);
-    payload += "\r\n\r\n";
-    payload += body;
-
-    return write(fd, payload.c_str(), payload.length()) != -1;
+    return write(fd, body.c_str(), body.length()) != -1;
 }
 
 void AdminClient::process()
@@ -94,14 +90,24 @@ void AdminClient::process()
 
     if (read_request(m_fd, request))
     {
+        string response;
         SHttpRequest parser(HttpRequest::parse(request));
-
-        enum http_code status = parser.get() ? HTTP_200_OK : HTTP_400_BAD_REQUEST;
-
         atomic_write_int64(&m_last_activity, hkheartbeat);
 
-        /** Echo the request body back */
-        write_response(m_fd, HTTP_200_OK, parser->get_json_str());
+        if (parser.get())
+        {
+            /** Valid request */
+            response = HttpResponse(parser->get_json_str()).get_response();
+        }
+        else
+        {
+            request = HttpResponse("", HTTP_400_BAD_REQUEST).get_response();
+        }
+
+        if (!write_response(m_fd, response))
+        {
+            MXS_ERROR("Failed to write response to client: %d, %s", errno, mxs_strerror(errno));
+        }
     }
     else
     {