FutureWarning raised by browser.cookies.delete()

[jace]

Pytest-Splinter's browser fixture deletes cookies by calling browser.cookies.delete():

pytest-splinter/pytest_splinter/plugin.py

Lines 609 to 615 in 47141cd

	try:
	browser.cookies.delete()
	except (IOError, HTTPException, WebDriverException):
	LOGGER.warning("Error cleaning browser cookies", exc_info=True)
	for url in splinter_clean_cookies_urls:
	browser.visit(url)
	browser.cookies.delete()

This is causing a FutureWarning under splinter 0.19:

…/splinter/driver/webdriver/cookie_manager.py:24: FutureWarning: Deleting all cookies via CookieManager.delete() with no arguments has been deprecated. use CookieManager.delete_all().

Splinter's documentation has been updated to recommend calling delete_all(): https://splinter.readthedocs.io/en/latest/cookies.html#delete-all-cookies

[jace]

I missed the existing open PR in #172.