Avoiding the crash when 'lastModified' is null

The crash was occurring in the method 'dictionaryWithObject' due to an 'NSInvalidArgumentException'
Using the same strategy of the Android version - setting the current time to the 'lastModified' when it's null
Merge pull request #8 from appglu/Avoiding_the_crash_when_lastModified_is_null

Avoiding the crash when 'lastModified' is null