XMLHttpRequest.open() should be correctly overloaded

PASS: if async argument is true, send() should behave asynchronously
PASS: if async argument is false, send() should behave synchronously
PASS: if async argument is not given, send() should behave like as async=true
PASS: if async argument is undefined, send() should behave like as async=false
PASS: if async argument is a non-empty string, send() should behave like as async=true