-[NSHTTPURLResponse allHeaderFields] returns a case-insensitive dictionary.

AllHeaderFields.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[])
{
	@autoreleasepool
	{
		NSDictionary *headerFields = @{ @"ETag" : @"12345678" };
		NSHTTPURLResponse *response = [[NSHTTPURLResponse alloc] initWithURL:[NSURL URLWithString:@"http://www.example.com"] statusCode:200 HTTPVersion:@"HTTP/1.1" headerFields:headerFields];
		NSLog(@"1. eTaG: %@", [[response allHeaderFields] objectForKey:@"eTaG"]);
		NSLog(@"2. eTaG: %@", [headerFields objectForKey:@"eTaG"]);
	}
}

1. eTaG: 12345678
2. eTaG: (null)