fix: don't create Headers object if headers are already of that type

[serozhenka]

The Headers.encoding gets stripped off in the BaseSession._set_curl_options method if the headers are already of type Headers. This PR aims to fix this by checking whether the headers are of the expected type.

[lexiforest]

That's indeed a bug, but what about fixing it in the __init__ method of Headers?

curl_cffi/curl_cffi/requests/headers.py

Lines 114 to 115 in 5074660

	elif isinstance(headers, Headers):
	self._list = list(headers._list)

The _encoding attribute is not copied from the headers passed in.

Anyway, it would be better to add a simple unittest in the test_headers.py file.

[serozhenka]

@lexiforest alrighty, will do it inside the Headers' __init__ it also makes more sense for me, and will add the unit test.