Dictionary object values should be utf-16

Set "PyPDF2" as an example for how I believe metadata should be encoded

It's in the PDF standard that all unicode metadata must be utf-16 (utf-8 is not okay, though most applications seem to tolerate it alright). All metadata must either be ASCII or utf-16, period. Unless I've missed the PDF revision that says otherwise, which is always possible.

Ideally, I think PyPDF2's metadata methods should accept unicode string and encode them as utf-16 before writing. Though I'd like feedback from the project management on this idea.