Passenv: Include default env variables regardless of their case on UNIX

[poggenhans]

This fixes problems with environment variables that are also used in their lower-case form on UNIX systems, such as http_proxy and https_proxy. They wouldn't be passed on to the test environment, while variables such as HTTP_PROXY would.

Closes #2372

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation, breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with -- by :user:`<your username>`.
  • please, use full sentences with correct case and punctuation, for example:

    Fixed an issue with non-ascii contents in doctest text files -- by :user:`superuser`.

  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[gaborbernat]