Remove test for null values in header map.
Remove test for null values in header map.

It is awkward to implment on Android and doesn't actually clobber
system-level headers anyways.

lorinbeer/cordova-mobile-spec,corimf/cordova-mobile-spec,polyvi/xface-mobile-spec,polyvi/xface-mobile-spec,apache/cordova-mobile-spec,ollie314/cordova-mobile-spec,corimf/cordova-mobile-spec,polyvi/xface-mobile-spec,apache/cordova-mobile-spec,corimf/cordova-mobile-spec,apache/cordova-mobile-spec,apache/cordova-mobile-spec,corimf/cordova-mobile-spec,ollie314/cordova-mobile-spec,apache/cordova-mobile-spec,ollie314/cordova-mobile-spec,ollie314/cordova-mobile-spec,ollie314/cordova-mobile-spec,corimf/cordova-mobile-spec