fix: keep RowIterator.total_rows populated after iteration

[tswast]

Split out of #1722

This was being reset in some cases when the rows were all available in the first page of results.

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Fixes #589 🦕

[tswast]

Alternatively, we could populate totalRows here if present:

python-bigquery/google/cloud/bigquery/table.py

Line 1703 in ab73796

response = {

Not sure which to prefer, but this way involved updating fewer tests.