Fix invalid utf8 decoding error handler in HeadersParser

[OttoWinter]

What do these changes do?

The xmlcharrefreplace error handler is invalid for decode operations (only encode):

In [1]: b = b"\xc3\x28"  # invalid utf8

In [2]: b.decode("utf8", "xmlcharrefreplace")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-0fb1f7b80b46> in <module>
----> 1 b.decode("utf8", "xmlcharrefreplace")

TypeError: don't know how to handle UnicodeDecodeError in error callback

In [3]: b.decode("utf8", "surrogateescape")
Out[3]: '\udcc3('

When these LineTooLong exceptions are triggered with non-utf8 input, a TypeError exception was raised

This was found by atheris

Are there changes in behavior for the user?

No.

Related issue number

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [ ] Documentation reflects the changes
• [x] If you provide code modification, please add yourself to CONTRIBUTORS.txt
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  • Please keep alphabetical order, the file is sorted by names.
• [x] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
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    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."