supporting the "start_index" argument in convert_request #3

These two lines should be added in the definition of convert_request, to support the start_index argument that is valid in a v3 request and supported (under a different name and type) in the v4 of the API. Sorry about telling you this in "issues", but I don't know how to actually use pull requests.

if 'start_index' in kwargs:
  report_request['pageToken'] = str(kwargs.get('start_index'))