Add 'middle' as a valid value for Table.props.size

[wms]

Dynamic settings at https://ant.design/components/table/#components-table-demo-dynamic-settings indicate that 'middle' is a valid option for Table.props.size, but this value is not valid according to the Table.Props interface, so I've added it.

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[mention-bot]

@warrenseymour, thanks for your PR! By analyzing the history of the files in this pull request, we identified @afc163, @benjycui and @yesmeck to be potential reviewers.

[coveralls]

Coverage remained the same at 82.481% when pulling dd16c70 on FountainheadTechnologies:add-middle-table-size into 16ad027 on ant-design:master.

[yesmeck]

Thanks.