Clarify string encoding when sending push notifications

[funkyboy]

Customer reported that some fields of push notifications (e.g. values _loc-key_ and _title-loc-key_) where received base64-encoded on iOS. Using our Python SDK (1.0.1) and Python 2.7

The issue was that they were sent like

'alert': {
  'title-loc-key': '{}-{}-NOTIFICATION-TITLE'.format(item_type, event).upper(),
  ...
}

whereas in Python 2 it should be

'alert': {
  'title-loc-key': u'{}-{}-NOTIFICATION-TITLE'.format(item_type, event).upper(),
  ...
}

so that the u makes sure the content is a unicode string.

While this is something that Python devs usually know, I think it's worth a mention in the README and the documentation about push notifications (when we'll write it).

[SimonWoolf]

Note that for the data payload of a normal message, we try and make this 'just work' by trying to coerce it to unicode, and sending it as binary data iff that results in a UnicodeDecodeError -- see https://github.com/ably/ably-python/pull/64/files . But I suspect that wouldn't be appropriate for push stuff given that the fields aren't fixed (at least not in a way the ably-python can be expected to know about), so we probably shouldn't be messing about with them. So yeah, better addressed by documentation

[d8x]

Assuming in Python 3 default storing of strings is unicode and there is no support for Python 2 I think we do not need to clarify it anymore and we can close the issue