Fix logging for transfer data sync_level
Fix logging for transfer data sync_level

Two problems with the existing implementation:

- Python 3 needs `next(generator)` rather than `generator.next()`
- if the SDK caller uses a `sync_level` that isn't in the dict, the use
  of a fallback argument in `sync_dict.get(sync_level, sync_level)` will
  make back to the user's input, which means that the use of the
  generator here will raise a `StopIteration`

Really, we should be able to just avoid the use of the generator
altogether by not clobbering the original `sync_level` that the caller
used and outputting it with what we map it to (`self['sync_level']`).

globus/globus-sdk-python,globus/globus-sdk-python,globusonline/globus-sdk-python,aaschaer/globus-sdk-python,sirosen/globus-sdk-python