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

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

_.toLower('--Foo-Bar--');
// => '--foo-bar--'

_.toLower('fooBar');
// => 'foobar'

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