Make feedparser accepts caseinsensitive headers

Currently FeedParserDict's headers type is dict.

Lines 195 to 200 in 0af72dc

    
           result = FeedParserDict( 
        
               bozo=False, 
        
               entries=[], 
        
               feed=FeedParserDict(), 
        
               headers={}, 
        
           )

But while assigning response_headers which is type of CaseInsensitiveDict to headers it act like dict.

feedparser/feedparser/api.py

Lines 224 to 227 in 0af72dc

    
           # overwrite existing headers using response_headers 
        
           result["headers"].update(response_headers or {}) 
        
           try:

If this acts like this it's hard to parse Content-Type

HTTP/1.1 200 OK
Connection: keep-alive
Date: Sat, 30 Dec 2023 09:20:21 GMT
Content-Type: text/xml; charset=UTF-8
Transfer-Encoding: chunked

If it is expected behavior, how can I modify Content-Type overriding Http requester?

FYI: requests headers type is CaseInsensitiveDict
https://github.com/psf/requests/blob/72eccc8dd8b7c272e520f22b0256386c80864e94/src/requests/adapters.py#L293-L314

	result = FeedParserDict(
	bozo=False,
	entries=[],
	feed=FeedParserDict(),
	headers={},
	)

	# overwrite existing headers using response_headers
	result["headers"].update(response_headers or {})

	try: