Tests: Times: JS: Add test for timestamp persistence.

tests/js/ttimes.nim

@@ -12,6 +12,14 @@ block yeardayTest:
   # check if yearday attribute is properly set on TimeInfo creation
   doAssert fromSeconds(2147483647).getGMTime().yearday == 18
 
-block timezoneTest:
+block localTimezoneTest:
   # check if timezone is properly set durint Time to TimeInfo conversion
   doAssert fromSeconds(2147483647).getLocalTime().timezone == getTimezone()
+
+block timestampPersistenceTest:
+  # check if timestamp persists during TimeInfo to Time conversion
+  const
+    testString = "2017-03-21T12:34:56+04:00"
+    fmt = "yyyy-MM-dd'T'HH:mm:sszzz"
+
+  doAssert $testString.parse(fmt) == testString