don't overwrite already existing env vars

[ccutrer]

this is helpful if your local env is different than real travis

for example, in real travis you have TEST_DB_USERNAME=postgres
in your travis.yml, but locally you have TEST_DB_USERNAME=cody
since you don't want (or have) a postgres superuser named postgres

[grosser]

there are things like RUBY_VERSION / BUNDLE_GEMFILE that need to changed even if they are currently set in your environment ... and I think guessing which ones will break an equal but opposite amount of usecases ;)

maybe a wwtd-ignore: section ?

[ccutrer]

BUNDLE_GEMFILE will be unset by Bundler.with_clean_env

[grosser]

maybe whitelist TEST_DB_USERNAME / other known ones only ?

[ccutrer]

hmm, I think you're saying that the env going to the sh command might include things like RUBY_VERSION from rvm/wwtd itself that rvm needs to execute properly; my intent is to only apply this logic for things that came from the travis.yml. what would you think to moving the filtering up to Run#add_env_from_config?

[grosser]

that would be a nicer place ... I still think there might be things you want to have overwritten ... so a explicit whitelist would be nice ... I think travis ignores extra keys, so they can live in the .travis.yml or be passed via arguments