[Fix #1449] Detect unrecognized style in MultilineOperationIndentation

It's when we forget to call unrecognized_style_detected that we
can get the strange Enabled: true setting in a .rubocop_todo.yml
file.
Merge pull request #1452 from jonas054/1449_MultilineOperationIndentation_todo

[Fix #1449] Detect unrecognized style in MultilineOperationIndentation