Cover images and pages not found if cover href has URL encoded characters

[ajbanck]

If a cover image or page has special characters in it's name PackageDocumentReader fails to set the cover.
This as PackageDocumentReader findCoverHrefs does not decode the URL-encoded hrefs to match the resources hrefs

So if the cover image href is cover%2B.png, findCoverHrefs returns 'cover%2B.png' not matching the 'cover+.png' resource

Example:
Cover image: cover+.png
opf manifest item: <item id="cover-image" href="cover%2B.png" media-type="image/png"/>

-> error 'PackageDocumentReader - Cover resource cover%2B.png not found'