Fix Text nodes being re-rendered unnecessarily

[developit]

During hydration, we always diff against an empty object as a stand-in for the old VNode. This means newVNode.props is never equal to oldVNode.props. However, hydration should never produce DOM mutations.

This PR addresses the newProps !== oldProps false positive by bypassing Text updates during hydration. It also adds an additional safeguard because in practise I've been seeing Text updates where the new text is exactly the same as the text contents present in the DOM tree. Assuming this happens reasonably often, it's cheaper to check for equality than it is to invalidate layout.

[coveralls]

Coverage remained the same at 100.0% when pulling 2d9339d on fix-text-always-rerendering into 45328e9 on master.