function toUpper(value) {
      return toString(value).toUpperCase();
    }
Converts `string`, as a whole, to upper case just like
[String#toUpperCase](https://mdn.io/toUpperCase).

@static
@memberOf _
@since 4.0.0
@category String
@param {string} [string=''] The string to convert.
@returns {string} Returns the upper cased string.
@example

_.toUpper('--foo-bar--');
// => '--FOO-BAR--'

_.toUpper('fooBar');
// => 'FOOBAR'

_.toUpper('__foo_bar__');
// => '__FOO_BAR__'