Fix incorrect ID generation for images in manifest

[bmaupin]

The media name was taken as the xml id for the manifest. This
breaks XML rules when for example the image file name

• starts with a number
• contains spaces or colons

[bmaupin]

Looks like there's one validation warning:

$ go test
Validating using EPUB version 3.2 rules.
WARNING(PKG-010): My EPUB.epub/EPUB/images/filename with space.png(-1,-1): Filename contains spaces, therefore URI escaping is necessary. Consider removing spaces from filename.

(https://github.com/bmaupin/go-epub/runs/2887794709?check_suite_focus=true)

[bmaupin]

I just made a change to the original commit that generates the exact same manifest IDs for testing but doesn't require any additional test files.

[bmaupin]

Looks like there's one validation warning:

$ go test
Validating using EPUB version 3.2 rules.
WARNING(PKG-010): My EPUB.epub/EPUB/images/filename with space.png(-1,-1): Filename contains spaces, therefore URI escaping is necessary. Consider removing spaces from filename.

Unfortunately it doesn't look like this warning can be fixed, even by escaping the URI: w3c/epubcheck#819

But I think we could fix it just by testing the filename with spaces outside of the EPUB validation test.

[bmaupin]

@pgundlach Ok, I was able to fix the validation warning by adding a new test, which has the added benefit of doing a more thorough check of the items in the package manifest. Writing it reminded me of why I don't like to write Go anymore, but it's done 😅

Thanks for your contribution and for your patience!