[Table] EditableCell + setState = value doesn't change

[seansfkelley]

Bug report

• Package version(s): whatever is on the docs at the time of filing (Core v1.20.0, Table v1.17.0)
• Browser and OS versions: Chrome 58.0.3029.110, OS X 10.12.5

Steps to reproduce

1. Go to the "Editable content" docs: http://blueprintjs.com/docs/#table-js.editable-content and open React dev tools
2. Find the TableEditableExample component and select it such that it's assigned to $r in the console
3. run $r.setState({ sparseCellData: Object.assign({}, $r.state.sparseCellData, { '1-1': 'foo' }) }) in the console

Actual behavior

1. Nothing changes.

Expected behavior

1. The value of the cell currently set to "editable" should change to "foo". Note that the dev tools reflect the correct state and the implementation of the example isn't doing anything particularly weird (https://github.com/palantir/blueprint/blob/master/packages/table/examples/tableEditableExample.tsx).

This is a stripped-down version of the issue I originally saw, but note that in my actual code, I use the intent field for validation as well, which is calculated at render time (as opposed to cached in state like the example), and changing the value using setState does cause the intent to change in the DOM, but not the text content (as here).

In my use-case, the table is editable and part of a controlled component, so every time you change something it makes the callback -> props roundtrip, which means I hit this issue all of the time. :(