Allowing for arbitrarily nested dictionaries in _log_entry_mapping_pb.

[dhermes]

Fixes #2552.

@waprin Do you think it's worthwhile to add a system test for this / update an existing system test with a nested dictionary?

[daspecster]

@dhermes, did you try this on the service and confirm that the data gets logged correctly?
I haven't found an issue but I'm curious if the upstream service captures the nested data properly.

[dhermes]

@daspecster

>>> from google.cloud import logging
>>> client = logging.Client()
>>> logger = client.logger('errors')
>>> 
>>> payload = {
...     'a': {
...         'b': 'c',
...     }
... }
>>> logger.log_struct(payload)
>>> 

[waprin]

@dhermes thanks for figuring this out for me, works for me. As for system test, I think it'd be trivial to add it just by adding one level of nesting to the current log_struct system test.

[dhermes]

@waprin Good find on ParseDict, updated it in the 2nd commit (and also updated the system test).

[waprin]

LGTM!