Use ThemeProvider theme instead of defaultProps theme - #345

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@k15a There's the test you wrote on #295 and I added a test in ebd4607 to ensure that props will take precedence over ThemeProvider even if its value is equal to defaultProps. Do you know another use case?

I made small changes on the code for better readability. Also added some comments (I'm not a big fan of it, but this props > ThemeProvider > defaultProps may be confusing).

This still dosen't work as it should be, check this example:
https://codesandbox.io/s/6w6nq15n2k

@ghalex Once you pass the prop from a normal component to a styled component ({...this.props}), there's no way for the latter to know whether it came from props or default props.

But you can enhance component with withTheme: https://codesandbox.io/s/m3nrmxvjw9

Thanks @diegohaz I understand now why it only works if you pass defaultProps only directly to styled component.

Another thing @diegohaz your example crashes in production when you have no ThemeProvider. Check this https://codesandbox.io/s/vr3q97or5 I have deployed it here: https://csb-vr3q97or5-ylvgjfxbld.now.sh/ and it crash. I think is related to this #1709