[TableHeaderColumn][TableRow][TableRowColumn] Remove props.key calls

[jakeboone02]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

Calls to props.key will throw a warning in React v15 (see here).

This commit removes calls to props.key in TableHeaderColumn, TableRow, and TableRowColumn. TableRow children are given an auto-generated key, and children of TableHeaderColumn and TableRowColumn can define their own keys as needed. As far as I can tell, passing a component's key prop down to the root element of that component is unnecessary, even if a key is required for the component.

This PR is referenced here: #3913 (comment)

[mbrookes]

Contributes to #3642.

[nathanmarks]

@mbrookes This looks good to me... however I had some issues with the complex example table demo in the docs. But it seems as though that example is even buggy on 0.14.4.

Can you confirm my findings? if so this can be merged.

[mbrookes]

@nathanmarks - yes, I've noticed that before, but was too afraid to go near it! 😄