Fix handling of defaults in Header objects

[cAtaman]

Fix the handling of defaults passed to the Headers constructor. It now has uniform behavior with headers added with Headers.add

This fix uses Headers.extend to add defaults as opposed to List.extend used previously. Using the Headers' extend method allows all internal checks and conversions to be done on the default values passed.

For an example, newlines are not allowed in headers.

    h1 = Headers([('X-Example', 'foo\r\n bar')])
    h2 = Headers()
    h2.add('X-Example', 'foo\r\n bar')

Before this fix h1 would get created while h2 would fail. Now they would both fail.

• fixes Inconsistent handling of newlines in Headers object #1608

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.