Keeping virtual properties out of the mapping

When virtual properties are injected to the mapping, serialize.js throws an error: `Cannot read property 'getters' of undefined`

This fix keeps virtual properties out of the cleantree.

Without this fix, any schema indexed with non-indexed virtual properties will throw an exception while being indexed.
excluding a virtual property from mapping
Merge pull request #75 from nadeeshacabral/master

Keeping virtual properties out of the mapping