Make sure we treat response headers from the server as case insensitive

On the mac, the browser modify the header names

Raven.Client.Silverlight/Connection/HttpJsonRequest.cs

@@ -182,8 +182,14 @@ private T ReadResponse<T>(Func<WebResponse> getResponse, Func<Stream, T> handleR
 				}
 			}
 
-			ResponseHeaders = response.Headers.AllKeys
-					.ToDictionary(key => key, key => (IList<string>)new List<string> { response.Headers[key] });
+			ResponseHeaders = new Dictionary<string, IList<string>>(StringComparer.InvariantCultureIgnoreCase);
+			foreach (var key in response.Headers.AllKeys)
+			{
+				ResponseHeaders[key] = new List<string>
+				{
+					response.Headers[key]
+				};
+			}
 			ResponseStatusCode = ((HttpWebResponse)response).StatusCode;
 
 			using (var responseStream = response.GetResponseStream())