fix Object.assign regression in JSXOpeningElement

Merged username-removed-568470 requested to merge kzc/buble:163 into master

Fixes #163 (closed)

Before fix:

$ echo '<MyThing two={"This Fails"} {...props}></MyThing>;' | bin/buble --objectAssign
React.createElement( MyThing, undefined({}, { two: "This Fails" }, props));

After fix:

$ echo '<MyThing two={"This no longer Fails"} {...props}></MyThing>;' | bin/buble --objectAssign
React.createElement( MyThing, Object.assign({}, { two: "This no longer Fails" }, props));