fix: case insensitive processing of docx and pptx internals

[pavelbazika]

Hi,

Sometimes, the case in Content-Types.xml and zipped file names do not match. Maybe not an issue on Windows, but is is an issue, when such file is searched in map with file names. (In particular, there was a directory dosProps, but in xml Overrides was docprops).

I've changed the processing to be case insensitive.

Regards