Allow multipart uploads when Content-Type has been set on on a Session

[gvengel]

It's currently impossible to use multipart file uploads with a Session when the Content-Type header has been set session wide. The problem is that the session header takes precedence over the header returned by _encode_files. See #6992.

This change adds support for the Content-Type header being explicitly set to None on a multipart file upload, signaling that the session header should be ignored. Similar to the pattern used by #1921.

Usage:

from requests import Session
from io import StringIO

fh = StringIO("Lobster Thermidor")

session = Session()
session.headers["Content-Type"] = "application/json"
session.post(
    "http://localhost:8000/",
    data={"menu": "Egg and bacon. Egg, sausage, and bacon. Egg and Spam"},
    files={"file": ("special.txt", fh, "text/plain")},
    headers={"Content-Type": None},  # unset the session content-type to allow multipart file upload
)

[gvengel]

Note, I think the "correct" fix is actually for multipart file uploads to always force the Content-Type, since the request must contain the disposition generated by _encode_files to remain valid; however, that would cause an implicate change, so I figured this patch requiring an explicit change from the caller might be safer. Let me know if you are open to forcing the header and I'll update the MR.