In the indent script force the reset of the 'indentexpr' setting

If I load an nginx file in vim, and vim detects it as another filetype, then it's possible that the `indentexpr` setting will be set. If I then manually switch to `set filetype=nginx`, then the old `indentexpr` will still remain and will mess up the indenting.

This pull request makes sure to reset the `indentexpr` setting