Remove empty string default for props.value

enables use to alternate between passing undefined  and empty string as props.value to clear the component input values (because {value: undefined} doesn’t get defaulted to empty stirng)

Author: acusti

src/ReactCodeInput.js

@@ -284,7 +284,6 @@ ReactCodeInput.defaultProps = {
   disabled: false,
   forceUppercase: false,
   fields: 4,
-  value: '',
   type: 'text',
   filterKeyCodes: [189, 190],
   filterChars: ['-', '.'],