Returns the value that corresponds to the given header field.
- iOS 13.0+
- macOS 10.15+
- Mac Catalyst 13.0+
- tvOS 13.0+
- watchOS 6.0+
The name of the header field you want to retrieve. The name is case-insensitive.
The value associated with the given header field, or
nil if no value is associated with the field.
In keeping with the HTTP RFC, HTTP header field names are case-insensitive.