added options of default image, added no response exception

GravatarWrapper.Test/UnitTest1.cs

@@ -9,7 +9,7 @@ namespace GravatarWrapper.Test
         [TestMethod]
         public void TestMethod1()
         {
-            var req = GravatarWrapper.Request("test@mctest.com", 400, true);
+            var req = GravatarWrapper.Request("test@mctest.com", 400, false);
             if (req != null)
             {
                 req.Save(@"c:\users\test\Desktop\bmp.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);

GravatarWrapper/Exceptions/NoResponseReturnedException.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace GravatarWrapper.Exceptions
+{
+    class NoResponseReturnedException : Exception
+    {
+        public NoResponseReturnedException()
+            : base("No response returned from server"){}
+    }
+}

GravatarWrapper/GravatarWrapper.cs

@@ -7,6 +7,8 @@ using System.Security.Cryptography;
 using RestSharp;
 using System.Drawing;
 using System.IO;
+using GravatarWrapper;
+using GravatarWrapper.Exceptions;
 
 namespace GravatarWrapper
 {
@@ -34,7 +36,7 @@ namespace GravatarWrapper
         {
             var client = new RestClient("http://www.gravatar.com/avatar/" + HashEmailForGravatar(email));
             var req = new RestRequest(Method.GET);
-            if (ShowDefaultImage) req.AddParameter("d", "%22%22" );
+            if (!ShowDefaultImage) req.AddParameter("d", "%22%22" );
             req.AddParameter("s", size);
             var res = client.Execute(req);
             if (res != null)
@@ -47,7 +49,7 @@ namespace GravatarWrapper
                 }
                 
             }
-            return null;
+            throw new NoResponseReturnedException();
         }
     }
 }