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

>>>fromgoogle.cloudimportlogging>>>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!