making IE specific declarations optional on XMLHttpRequest interface

_XMLHttpRequest_ interface has IE only fields like _msCaching_ but they are not declared as optional with _msCaching?_ syntax. Is there any reason behind of it? I believe they should be declared as optional.
refer https://github.com/Microsoft/TypeScript/commit/1dacb54fbde2c2af772326981e6d753f76b1955a#diff-b32ebd70e111503a0b6d629131e5acdbR775

thank you.
