When only one TableHeaderColumn in table export fails

On version 4.3.1 the export function in BootstrapTable.js is as follows: `this.props.children.filter(_` => _ != null).map(function(column)`

Unfortunately `this.props.children` is only an array when there are more than one column configured. When there is only one it is no longer an array, but instead it is an object. I think that since the property is called `children`, which implies an array more than one item, it should be an array with only one item when there is only one column. I'm not sure if this `children` is being supplied by react so it may need to be fixed with a `if` check instead.

Here is a link to the current code in github: https://github.com/AllenFang/react-bootstrap-table/blob/26d07defab759e4f9bce22d1d568690830b8d9d7/src/BootstrapTable.js#L1174