fix(Accordion): respect defaultActiveIndex

[levithomason]

Fixes #761

The defaultActiveIndex is overridden in the Accordion on componentWillMount. This PR adds an inline manual check for the default prop and a TODO to resolve the bug. I've also added 2 tests to assert the initial index of -1 and the default prop works correctly.

This is an edge case, but we need to update the AutoControlledComponent to defer to default props when calling trySetState in componentWillMount.

The real solve here is likely adding static defaultAutoControlledProps. Currently, the AutoControlledComponent tells devs to use trySetState in componentWillMount instead of setting defaults for auto controlled props. This is to prevent the actual defaults from always winning, however, if we had our own auto controlled defaults then we could apply them correctly.

[codecov-io]

Current coverage is 99.49% (diff: 100%)

Merging #763 into master will increase coverage by <.01%

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