Sortable - fixed parentOffset calculation 

In `Sortable` plugin in the `_getParentOffset` method there is a bit of code described as "a special case where we need to modify a offset calculated on start" which adds `scrollParent.scrollTop()` to the `parentOffset.top`.

It does it if (among others) `scrollParent` is not equal to `document`. On chrome the condition was evaluating to true even if `scrollParent` was `document.body`; This pull request adds necessary check for `document.body`.