add warning when timeout without total is used on App Engine
add warning when timeout without total is used on App Engine

motivated by sigmavirus24/requests-toolbelt#146. cc @jonparrott.

Lukasa/urllib3,urllib3/urllib3,Disassem/urllib3,urllib3/urllib3,Disassem/urllib3,Lukasa/urllib3,haikuginger/urllib3,sigmavirus24/urllib3,haikuginger/urllib3,sigmavirus24/urllib3