feat: allow custom headers in urlopener + pass to it from urlopen

[seemab-yamin]

What this does
Allows passing custom headers directly to urlopener(headers=...) (previously it always used only url_default_headers).
Also propagates user headers from urlopen/urlread/etc → urlopener, so custom headers can override or add to defaults.

Why
Before, even if you passed headers= to urlopen, they only went to the Request object — the opener-level defaults were never combined or overridable per-call.

Repro before fix (fails to send custom headers at opener level)

from fastcore.net import urlread

url = "https://httpbin.org/headers"
headers = {"X-Custom-Header": "should-appear"}

resp = urlread(url, headers=headers)
print(resp)  # X-Custom-Header missing from response (only defaults applied)

[seemab-yamin]

@jph00 - Please review and let me know if there is any fix or style improvement. Thanks for your time.