Fix incorrect plus(Long,...) behavior in js-joda

When 0L was passed as JS number argument, it failed to equal strictly
to zero number, thus triggering an incorrect execution path.

Author: ilya-g

core/commonTest/src/LocalDateTest.kt

@@ -82,6 +82,8 @@ class LocalDateTest {
     @Test
     fun diffInvariant() {
         val origin = LocalDate(2001, 1, 1)
+        assertEquals(origin, origin.plus(0, DateTimeUnit.DAY))
+        assertEquals(origin, origin.plus(DatePeriod(days = 0)))
 
         repeat(1000) {
             val days1 = Random.nextInt(-3652..3652)

core/jsMain/src/LocalDate.kt

@@ -49,7 +49,7 @@ private fun LocalDate.plusNumber(value: Number, unit: CalendarUnit): LocalDate =
         }.let(::LocalDate)
 
 public actual fun LocalDate.plus(value: Long, unit: CalendarUnit): LocalDate =
-        plusNumber(value, unit)
+        plusNumber(value.toDouble(), unit)
 
 public actual fun LocalDate.plus(value: Int, unit: CalendarUnit): LocalDate =
         plusNumber(value, unit)