Bug 918721 - flip the relevant pref from bug 669259 and mark the relevant web platform test as passing. r=dragana

modules/libpref/init/all.js

@@ -1556,7 +1556,7 @@ pref("network.http.signed-packages.enabled", false);
 // array - behavior as it used to be. If it is true: empty headers coming from
 // the network will exist in header array as empty string. Call SetHeader with
 // an empty value will still delete the header.(Bug 6699259)
-pref("network.http.keep_empty_response_headers_as_empty_string", false);
+pref("network.http.keep_empty_response_headers_as_empty_string", true);
 
 // default values for FTP
 // in a DSCP environment this should be 40 (0x28, or AF11), per RFC-4594,

testing/web-platform/meta/XMLHttpRequest/getresponseheader-cookies-and-more.htm.ini

@@ -1,5 +0,0 @@
-[getresponseheader-cookies-and-more.htm]
-  type: testharness
-  [XMLHttpRequest: getResponseHeader() custom/non-existent headers and cookies]
-    expected: FAIL
-