Simplified ReactDOMComponent children logic

I found it quite hard to convince myself that the logic of `_createContentMarkup` and `_updateDOMChildren` didn't have inconsistencies for edge-cases. So I decided to simplify and homogenize the implementations so that they are more easily understood, IMHO.

Test plan: `grunt test`

```
   raw     gz Compared to master @ 57de1b07d0b58d5a76d46ee8b429d7d74d175c99
  -269    -46 build/react-with-addons.js
  -102    -22 build/react-with-addons.min.js
  -269    -48 build/react.js
  -102    -24 build/react.min.js
```