fix parameter reference in anonymous function

Would occasionally generate exception "Invalid attempt to call MetaData when reader is closed"

Dapper/SqlMapper.cs

@@ -1524,7 +1524,7 @@ public object SetValue(string key, object value)
                         string[] names = new string[effectiveFieldCount];
                         for (int i = 0; i < effectiveFieldCount; i++)
                         {
-                            names[i] = reader.GetName(i + startBound);
+                            names[i] = r.GetName(i + startBound);
                         }
                         table = new DapperTable(names);
                     }