Fixing the url parameter encoding replace

Author: alexeyzimarev

src/RestSharp/Parameter.cs

@@ -33,7 +33,7 @@ public Parameter(string name, object value, ParameterType type)
                 Ensure.NotEmpty(name, nameof(name));
 
             Name  = name;
-            Value = type != ParameterType.UrlSegment ? value : value.ToString().Replace("%2F", "/").Replace("%2f", "/");
+            Value = type != ParameterType.UrlSegment ? value : value?.ToString().Replace("%2F", "/").Replace("%2f", "/");
             Type  = type;
         }
 

src/RestSharp/Validation/Ensure.cs

@@ -30,5 +30,12 @@ public static void NotEmpty(string parameter, string name)
             if (string.IsNullOrWhiteSpace(parameter))
                 throw new ArgumentNullException(name);
         }
+        
+        public static void NotEmptyString(object parameter, string name)
+        {
+            var s = parameter as string;
+            if (string.IsNullOrWhiteSpace(s))
+                throw new ArgumentNullException(name);
+        }
     }
 }