implement setCookies with mainDocumentURL

Author: Pushkar Kulkarni

Foundation/NSHTTPCookieStorage.swift

@@ -226,17 +226,22 @@ open class HTTPCookieStorage: NSObject {
         in accordance with policy settings.
     */
     open func setCookies(_ cookies: [HTTPCookie], for url: URL?, mainDocumentURL: URL?) {
+        //if the cookieAcceptPolicy is `never` we don't have anything to do
         guard cookieAcceptPolicy != .never else { return }
-        guard let theUrl = url else { return }
 
-        var validCookies = [HTTPCookie]()
-        if mainDocumentURL != nil && cookieAcceptPolicy == .onlyFromMainDocumentDomain {
-            //TODO: needs a careful study of the behaviour on Darwin
-            NSUnimplemented()
-        } else {
-            validCookies = cookies.filter { theUrl.host != nil && theUrl.host!.hasSuffix($0.domain) }
-        }
+        //if the urls don't have a host, we cannot do anything
+        guard let urlHost = url?.host else { return }
+        guard mainDocumentURL != nil, let mainDocumentHost = mainDocumentURL!.host else { return}
+
+        //if the mainDocumentURL is non-nil then the cookieAcceptPolicy must be `onlyFromMainDocumentDomain` only
+        guard mainDocumentURL != nil && cookieAcceptPolicy == .onlyFromMainDocumentDomain else { return }
+
+        //if the mainDocumentURL is non-nil, the url.host must be a suffix of manDocumentURL.host
+        //this is based on Darwin's behaviour
+        guard mainDocumentURL != nil && mainDocumentHost.hasSuffix(urlHost) else { return }
 
+        //save only those cookies whose domain matches with the url.host
+        let validCookies = cookies.filter { urlHost == $0.domain }
         for cookie in validCookies {
             setCookie(cookie)
         }