Get header value by a case insensitive key

HTTPURLResponse+CaseInsensitiveHeaderField.swift

import Foundation

public extension HTTPURLResponse {
    /// Return http header value for case insensitive `headerField` key
    ///
    /// Why do we need this?
    /// Well, there is a bug, which leads `allHeaderFields` dictionary to be case sensitive.
    /// https://bugs.swift.org/browse/SR-2429
    /// Workaround with casting to `NSDictionary` is still working, but Apple introduced a new API to get header value by a key
    /// https://developer.apple.com/documentation/foundation/httpurlresponse/3240613-value
    func valueForHeaderField(_ headerField: String) -> String? {
        if #available(iOS 13.0, *) {
            return value(forHTTPHeaderField: headerField)
        } else {
            return (allHeaderFields as NSDictionary)[headerField] as? String
        }
    }
}