Update SqlMapperExtensions.Async.cs

#418 - `Update` was fixed but `UpdateAsync` was not fixed.

Dapper.Contrib/SqlMapperExtensions.Async.cs

@@ -232,7 +232,7 @@ public static partial class SqlMapperExtensions
                 }
             }
 
-            var keyProperties = KeyPropertiesCache(type);
+            var keyProperties = KeyPropertiesCache(type).ToList();
             var explicitKeyProperties = ExplicitKeyPropertiesCache(type);
             if (keyProperties.Count == 0 && explicitKeyProperties.Count == 0)
                 throw new ArgumentException("Entity must have at least one [Key] or [ExplicitKey] property");