fix: instance locale change should be immutable

Author: iamkun

src/index.js

@@ -375,11 +375,15 @@ class Dayjs {
     return Ls[this.$L]
   }
 
-  locale(preset, object) {
+  $setLocale(preset, object) { // private set locale mutate instance
     this.$L = parseLocale(preset, object, true)
     return this
   }
 
+  locale(preset, object) {
+    return this.clone().$setLocale(preset, object)
+  }
+
   clone() {
     return wrapper(this.toDate(), this)
   }

test/locale.test.js

@@ -49,6 +49,20 @@ it('set global locale', () => {
     .toBe('Saturday 28, April')
 })
 
+it('immutable instance locale', () => {
+  dayjs.locale('en')
+  const origin = dayjs('2018-4-28')
+  expect(origin.format(format))
+    .toBe('Saturday 28, April')
+  expect(origin.locale('es').format(format))
+    .toBe('Sábado 28, Abril')
+  const changed = origin.locale('es')
+  expect(changed.format(format))
+    .toBe('Sábado 28, Abril')
+  expect(origin.format(format))
+    .toBe('Saturday 28, April')
+})
+
 it('User custom locale', () => {
   expect(dayjs('2018-4-28')
     .locale('xx', {