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
Merge pull request #375 from danielmiladinov/master

Change spec policy for getDefaultProps to SpecPolicy.DEFINE_MANY_MERGED