selectfieldset: fix select style

[joelguittet]

Hello

We can see on the selectfieldsetexample (https://jsonform.github.io/jsonform/playground/index.html?example=fields-selectfieldset) that the select field used to perform the selection between the item has not the proper style:

To avoid complex style with css stuff, I suggest to add the class "form-control" to the select field generated by the lib. See git diff of this PR.

Tested with my own project OK but I let you check it and verify it has no unwanted impact, I guess there isn't.

Joel

[tchapi]

Thanks a lot @joelguittet !

[joelguittet]

Thanks for being so quick to get it merged !
I see the example is also updated, looks nice now :-)

Joel