Fixed HTTPSession to use request sessions so that connection pool is not recreated upon every request to a spreadsheet.
Fixed HTTPSession to use request sessions so that connection pool is not recreated upon every request to a spreadsheet.

Without this fix, you see following log message on every request to a spreadsheet:

...INFO|requests.packages.urllib3.connectionpool|connectionpool.py:756|_new_conn()|Starting new HTTPS connection (1): spreadsheets.google.com

Creationg HTTPS connections is expensive. So this fix impoves the overall performance significantly.

See issue #380.

nateyoder/gspread,wbrp/gspread,burnash/gspread,ShivaShinde/gspread