[styles] Theme default props are overriden by Component default props

[adridavid]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Using withStyles, theme default props are overriden by Component default props.

// Component
import Button from "@material-ui/core/Button";

const MyButton = props => <Button {...props} />;
MyButton.defaultProps = { variant: "outlined" };
const MyStyledButton = withStyles({}, { name: "MyButton" })(MyButton);

// Theme
const theme = createMuiTheme({
  props: {
    MyButton: {
      variant: "contained"
    }
  }
});

In the example above, the button will be outlined. It should be contained.

Moreover we don't have the same behavior if we use props destructuring with default values instead of defaultProps. In this case, the button would be contained.
Theme props work as expected for Material-UI components because they use props destructuring.

Expected Behavior 🤔

Theme default props should override Component default props.

Steps to Reproduce 🕹

A code sandbox example is available here.