…de_header()

This function's possible return types have been surprising and error-prone
for the entirety of its Python 3.x history. It can return either:

1. `typing.List[typing.Tuple[bytes, typing.Optional[str]]]` of length >1
2. or `typing.List[typing.Tuple[str, None]]`, of length exactly 1

This means that any user of this function must be prepared to accept either
`bytes` or `str` for the first member of the 2-tuples it returns, which is a
very surprising behavior in Python 3.x, particularly given that the second
member of the tuple is supposed to represent the charset/encoding of the
first member.

This patch documents the behavior of this function, and adds test cases
to demonstrate it.

As discussed in bpo-22833, this cannot be changed in a backwards-compatible
way, and some users of this function depend precisely on the existing
behavior.

This function takes an email header, possibly with portions encoded
according to RFC2047, and converts it to a standard Python string.

It is intended to provide a sane, Pythonic replacement for
`email.header.decode_header()`, which has two major problems:

1. May return either bytes or str (bpo-22833/pythongh-67022), an
   inconsistent and error-prone interface
2. Exposes details of an email header value's encoding which
   most users will not care about or want to deal with. Many users
   likely just want to decode an email header value to a Python
   string.

It turns out that `email.header` already contained most of the code
necessary to do this, and providing `decode_header_to_string` as a
documented wrapper function points users in the right direction.