got warning message when editable property is function which return false

When:
- editable property of TableHeaderColumn is a function
- the function returns false value

Then:
there is a warning message in console.

```
warning.js?6327:33 Warning: Received `false` for a non-boolean attribute `type`.

If you want to write it to the DOM, pass a string instead: type="false" or type={value.toString()}.

If you used to conditionally omit it with type={condition && value}, pass type={condition ? value : undefined} instead.
in input (created by TableEditColumn)
in td (created by TableEditColumn)
in TableEditColumn (created by TableBody)
in tr (created by TableRow)
in TableRow (created by TableBody)
in tbody (created by TableBody)
in table (created by TableBody)
in div (created by TableBody)
in TableBody (created by BootstrapTable)
in div (created by BootstrapTable)
in div (created by BootstrapTable)
in BootstrapTable (created by DocumentList)
in div (created by DocumentList)
```

this line sets ``

https://github.com/AllenFang/react-bootstrap-table/blob/ec9f036698362a5e4296920049ad3283ee66356b/src/Editor.js#L14