Change spec policy for getDefaultProps to SpecPolicy.DEFINE_MANY_MERGED

This will allow multiple mixins for a component to define
getDefaultProps and their values will be merged.

See also:
https://groups.google.com/d/msg/reactjs/UzSiXw2Vo5s/FxK7AHWOzLMJ

I work together with Dustin and we are both enjoying using React this was a question I initially posed to him when I came across the error message during development; he posted the topic here and encouraged me to submit this pull request after receiving your tacit approval to do so.