ev-document-factory: fix get_backend_info_for_mime_type for macOS and Windows

This patch fixes opening files when Evince is built against GLib containing GContentType implementations for non-Linux operating systems such as macOS and Windows. On such platforms content type is not equal to mime type, as stipulated in the GContentType documentation.

Without this patch, one gets the following error:

I first erroneously thought that this was a bug within Glib, but comments of the maintainers there led me to dig a bit deeper into the Evince code and I managed to come with a proper fix.