wsgiheaders: make sure __repr__() returns a string
When printing `req.headers` on the server side to debug, it complained that '%b'
needed to take a string, not bytes. Changing '%s' to '%r' caused it to complain
that __repr__ didn't return a string.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).