Fix for picker

[markterence]

This is a fix for issue #16 and #12.

The code causes the error. picker/index.js#L27

Object.assign(styles.pickerMainAndroid, {
 backgroundColor: theme.pickerBgColor,
 borderBottomColor: theme.inputBorderColor,
 borderBottomWidth: theme.borderWidth,
})

I found two ways to prevent this error.

Error : You attempted to set the key backgroundcolor with the value "transparent"
on an object that is meant to be immutables and has been frozen.

A. Using Spread Operator (...) (This is what i used)

<View
  style={{...styles.pickerMainAndroid, ...{
    backgroundColor: theme.pickerBgColor,
    borderBottomColor: theme.inputBorderColor,
    borderBottomWidth: theme.borderWidth,
 }}}
>

B. Using Object.assign()

• The first parameter should be an empty object.

<View
 style={Object.assign({}, styles.pickerMainAndroid, {
   backgroundColor: theme.pickerBgColor,
   borderBottomColor: theme.inputBorderColor,
   borderBottomWidth: theme.borderWidth,
})}
>