Fix bug introduced in c615478e

In c615478e, we moved from email.policy.default to email.policy.compat32.

As told in the documentation, email.policy.compat32 represents an email using an email.message.Message class:

>>> type(email.parser.BytesParser(policy=email.policy.compat32).parsebytes("Hey".encode("utf-8")))
<class 'email.message.Message'>

Whereas email.policy.default represents an email using an email.message.EmailMessage class:

>>> type(email.parser.BytesParser(policy=email.policy.default).parsebytes("Hey".encode("utf-8")))
<class 'email.message.EmailMessage'>

So, the unit tests should not test erine_email Python package using email.message.EmailMessage but email.message.Message.

Also, we should consider that, in a email.message.Message, a header can be either a string or an email.header.Header:

>>> m = email.message.Message()
>>> m["Subject"] = "Hey"
>>> h = email.header.Header()
>>> h.append("Hey".encode("utf-8"), "utf-8")
>>> m["Subject"] = h

Contrarily to the email.message.EmailMessage:

>>> m = email.message.EmailMessage()
>>> m["Subject"] = h
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/email/message.py", line 409, in __setitem__
    self._headers.append(self.policy.header_store_parse(name, val))
  File "/usr/lib/python3.9/email/policy.py", line 148, in header_store_parse
    return (name, self.header_factory(name, value))
  File "/usr/lib/python3.9/email/headerregistry.py", line 601, in __call__
    return self[name](name, value)
  File "/usr/lib/python3.9/email/headerregistry.py", line 196, in __new__
    cls.parse(value, kwds)
  File "/usr/lib/python3.9/email/headerregistry.py", line 271, in parse
    kwds['parse_tree'] = cls.value_parser(value)
  File "/usr/lib/python3.9/email/_header_value_parser.py", line 1112, in get_unstructured
    if value[0] in WSP:
TypeError: 'Header' object is not subscriptable

Note that part of the code proposed in this merge request come from before 3eb58026, when we moved from email.message.Message() (generated by email.message_from_binary_file()) to email.message.EmailMessage() (generated by email.parser.BytesParser(policy=default).parse()).

Unit tests passed. The coverage code on the erine_email Python package is still 100%.