EPUB 2 meta tags should be superseded by EPUB 3 ones

[mickael-menu]

When an OPF contains both an EPUB 2 (<meta name=>) and EPUB 3 (<meta property=>) meta tags for the same key, the EPUB 3 one should take precedence and the other one ignored. Currently both are added to the otherMetadata dictionary.

Here's an example:

<meta property="media:active-class">-epub-media-overlay-active</meta>
<meta name="media:active-class" content="text-highlight" />

See the decision in the last meeting minutes.