Offset: don't run scrollTop/scrollLeft iframe test in mobile Safari

iPhone resize the iframe by its content regardless of the width, height values,
meaning it's not possible to scroll the iframe only its parent element

Ref 62a333e0646d3011377ed13a6fcfbb08e91e2bef
Ajax: simplify jQuery.parseXML method

-2 bytes to the gzipped size and code looks a bit nicer

This wasn't landed to the compat branch though, since size would increase
with this cherry-pick

Closes gh-1856
Deferred: Fix $.when with resolved deferred and progress callbacks

Fixes gh-1894
Closes gh-1915