TabSelect sometimes breaks when only one tab is passed

[fbarl]

Probably due to the this.props.children structure when a singleton is passed down (probably not in the form of array).

Encountered when trying it on https://github.com/weaveworks/service-ui/pull/2647.

cc @jpellizzari

[jpellizzari]

@fbarl What is the symptom of the breaking?

[fbarl]

So for a single child in the example:

<TabSelect>
  <Tab label="A" name="a">
    Tab A
  </Tab>
</TabSelect>

the whole page crashes with