0.7.0 duplicate plugin use causes failures comparing to undefined

[wbyoung]

Similar to #177 (or perhaps exactly the same issue), the following fails for me when a second run is performed in (mocha) watch mode:

expect(wrapper.find('span')).to.have.attr('id')

This can easily be reproduced by including the plugin twice in a testing setup (or even by updating the helpers.js to include the plugin twice in the chai-enzyme test suite).