Modification is made even if value is 'unset'

[fchiba]

According to the document, 'unset' should be treated as unspecified. But, actually 'unset' has some meanings on each property.

For example, if unset is specified for insert_final_newline, it's treated as truthy and newline is inserted. cf. https://github.com/slang800/editorconfig-tools/blob/master/lib/rules/insert-final-newline.coffee#L21

[fchiba]

https://github.com/slang800/editorconfig-tools/pull/38