Give safe_unicode a more correct name

[philippjfr]

The safe_unicode utility is very important in ensuring that bytes types are correctly cast to unicode but it does not safely cast all types to unicode as the name implies. We should come up with a more appropriate name.

[jlstevens]

Thanks for filing this. I noticed the confusing name when reviewing a PR but I haven't managed to come up with a better name yet. It isn't 'safe' as there was an exception raised when using the built in unicode() was enough - what exactly does it do again?

[philippjfr]

It safely casts str types with various encodings to unicode in Python 2 but leaves non string types unaffected.

[jlstevens]

So it goes from string encodings to unicode? Sounds like something I would expect to find in the standard library....

[jbednar]

unicodify? ensure_unicode? to_unicode? promote_to_unicode?