Added the additional test cases for PR #505 and excluded Silverlight from the change in RestRequest.

Author: Michael Hallett

RestSharp.Tests/RestRequestTests.cs

@@ -41,6 +41,8 @@ public void Cannot_Set_Empty_Host_Header()
 		[InlineData(".")]
 		[InlineData(".:2345")]
 		[InlineData(":5678")]
+		[InlineData("")]
+		[InlineData("foo:bar:baz")]
 		public void Cannot_Set_Invalid_Host_Header(string value)
 		{
 			var request = new RestRequest();

RestSharp/RestRequest.cs

@@ -357,12 +357,15 @@ public IRestRequest AddParameter (string name, object value, ParameterType type)
 		/// <returns></returns>
 		public IRestRequest AddHeader (string name, string value)
 		{
+#if !SILVERLIGHT
 			const string portSplit = @":\d+";
 			Func<string, bool> invalidHost = host => Uri.CheckHostName(Regex.Split(host, portSplit)[0]) == UriHostNameType.Unknown;
+
 			if (name == "Host" && invalidHost(value))
 			{
 				throw new ArgumentException("The specified value is not a valid Host header string.", "value");
 			}
+#endif
 			return AddParameter(name, value, ParameterType.HttpHeader);
 		}