Update RadioButtonGroup to allow setting an initialValue

[m14t]

Hi Erik,

First thank you for both redux-form and this wrapper library. I'm currently implementing a complex form that I need to know if it is "pristine" or not, and these libraries have been a huge time saver for me!

While working on this, I noticed that the initialValues were not getting set for RadioButtonGroup.

It turns out that this is because RadioButtonGroup expects the selected value to be passed in as valueSelected, not value.

This pull-request maps that value.

I tried to match your coding style as close as possible, but please leave me feedback, and I'll gladly make any changes you wish.

Thanks in advance!
-Matt

[m14t]

Thanks @erikras ! Would it be possible to get a tagged/npm release with these recent merges?

[m14t]

@erikras Sorry to pester, but it would still be great to see a release with the recent merges if possible.

Thanks again for all of your work here.

[erikras]

Published in v4.1.0.