Adding wrapper for tests that change uri ENV variable
minor: add back deleted line to env uri test
DRIVERS-133 Test that embedded doc keys are checked for $ with check_keys
minor: make sure env uri is reset on tests

use with_preserved_env_uri method in remaining tests that were still setting MONGODB_URI
minor: Use uri var to make test cleaner
Merge branch 'gjmurakami-10gen-1.x-stable-ruby-676' into 1.x-stable