headers are ignored when providing a custom StacIO

[gadomski]

In Client.open, the headers argument is ignored if at stac_io is provided:

pystac-client/pystac_client/client.py

Lines 194 to 199 in 37b9b59

	if stac_io is None:
	stac_io = StacApiIO(
	headers=headers,
	parameters=parameters,
	request_modifier=request_modifier,
	)

This can lead to surprising behavior when, e.g., using basic authentication with a custom StacIO:

from pystac_client import Client
from pystac_client.stac_api_io import StacApiIO

headers = {"Authorization": "Basic not-a-real-token"}
stac_io = StacApiIO()  # needs to be StacApiIO(headers=headers)
stac_io.session.verify = False
client = Client.open("https://pystac-client.test", headers=headers, stac_io=stac_io)  # headers are silently ignored

This should be fixed to either:

1. Set headers onto the stac_io, or
2. Warn/error if headers and stac_io are provided

Option 1 feels right, but let's see which implements better.