generateConfig for date-fns throws error for some locales

The generateConfig methods use dealLocal() function to get date-fns specific locale data for the provided language, but the implementation fails in some cases because rc-picker locales names do not exactly match with date-fns ones.

For example Italian:

• rc-picker use it_IT as locale name;
• dealLocal() converts it into itIT;
• generateConfig.getWeekFirstDay() method tries to load date-fns locale data with an itIT key, but it does not exists and this leads to clone constant to be undefined.

The only solution is to replace the current dealLocal() implementation with a static map of locales names and prevent errors with a default value:

const DEFAULT_LOCALE = 'enUS';

const LOCALES_MAP = {
  'en_US': 'enUS',
  'it_IT': 'it',
  'es_ES': 'es',

  // and so on...
}

const dealLocal = (str: string): string => LOCALES_MAP[str] || DEFAULT_LOCALE;

[stepanzin]

Same problem with ru_RU locale

[Saarnaki]

@zombieJ are there any issues regarding spawnia’s PR?