Improve interaction with -u

[dannystaple]

Currently if activate is sourced from within a script that uses "set -u" to detect unset variables, it will fail as shown in #150.
This proposed change should fix it.

[ppannuto]

@pypa Is there a reason this was never pulled? This is still a problem... just got bitten by this today.

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow pip has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master. Unfortunately, this pull request does not cleanly merge against the current master branch.

If you do nothing, this Pull Request will be automatically closed by @BrownTruck since it cannot be merged.

If this pull request is still valid, please rebase it against master (or merge master into it) and resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to rebase/merge and resubmit this Pull Request, here is an example message that you can copy and paste:

Currently if activate is sourced from within a script that uses "set -u" to detect unset variables, it will fail as shown in #150.
This proposed change should fix it.

---

*This was migrated from pypa/virtualenv#357 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

[BrownTruck]

This Pull Request was closed because it cannot be automatically reparented to the master branch and it appears to have bit rotted.

Please feel free to re-open it or re-submit it if it is still valid and you have rebased it onto master or merged master into it.