Don't throw on the empty content type

Return the first handler is only one is available.

Author: alexeyzimarev

RestSharp/RestClient.cs

@@ -476,12 +476,12 @@ private static IEnumerable<Parameter> GetQueryStringParameters(IRestRequest requ
 
         private Func<IDeserializer> GetHandler(string contentType)
         {
-            if (contentType == null)
-                throw new ArgumentNullException(nameof(contentType));
-
-            if (string.IsNullOrEmpty(contentType) && ContentHandlers.ContainsKey("*"))
+            if (contentType.IsEmpty() && ContentHandlers.ContainsKey("*"))
                 return ContentHandlers["*"];
 
+            if (ContentHandlers.Keys.Count == 1) 
+                return ContentHandlers.First().Value;
+
             int semicolonIndex = contentType.IndexOf(';');
 
             if (semicolonIndex > -1)