Enhance type of headers.get method.

[pcorpet]

When given a default value that is not None, the get method always returns a string.

• fixes Stronger typing for request.headers.get method #2128

I haven't seen any tests that checks for the types themselves, although I've changed one instance of usage in the werkzeug code itself that would fail without the typing change.

Checklist:

• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[ThiefMaster]

This should probably be rebased against 2.0.x (and the upstream branch changed here in the PR's settings)

[pcorpet]

Thanks @ThiefMaster , I hadn't seen that there was a new dev version in the works.

[davidism]

It seems you've made this PR from an organization account instead of your own, which means I can't push to it to prepare it for merge. Try to avoid this in the future, it's more convenient for maintainers to be able to modify PRs in place.

[pcorpet]

@davidism , sorry about that. I thought I had set the "maintainers can modify" flag. I'll be careful next time

[davidism]

From my experience, the setting doesn't seem to work on org prs.