Fix PropTypes warning, if using custom TextField. (#965)
Fix PropTypes warning, if using custom TextField. (#965)

When using the classical TextField comp. from Material-UI, wrapping it withStyles, results in an object. That is new, but ok.
Therefore extending the PropTypes here, so that warning does not accidentally happen anymore.
callemall/material-ui,rscnt/material-ui,rscnt/material-ui,mui-org/material-ui,mbrookes/material-ui,callemall/material-ui,callemall/material-ui,mbrookes/material-ui,dmtrKovalenko/material-ui-pickers,mbrookes/material-ui,oliviertassinari/material-ui,oliviertassinari/material-ui,oliviertassinari/material-ui,dmtrKovalenko/material-ui-pickers,mui-org/material-ui,mui-org/material-ui,callemall/material-ui,rscnt/material-ui