Fixes #80 - Fix when ObjectExpression properties don't have the 'Property' type

By (ab)using the generateExpression option object, so the ObjectExpression case
sets the expression type for its properties, when those are not set. This
prevents breakage, because according to SpiderMonkey Parse API, the type field
is not really required.
Merge pull request #81 from arian/fix-80-objectexpression

Fixes #80 - Fix when ObjectExpression properties don't have the 'Property' type