Fixes ConcurrentModificationException mentioned in Issue #47

[gwilli]

This crash was our app's most common crash prior to making this change.

Fatal Exception: java.util.ConcurrentModificationException
       at java.util.HashMap$HashIterator.nextEntry(HashMap.java:851)
       at java.util.HashMap$KeyIterator.next(HashMap.java:885)
       at com.franmontiel.persistentcookiejar.cache.SetCookieCache$SetCookieCacheIterator.next(SetCookieCache.java:67)
       at com.franmontiel.persistentcookiejar.cache.SetCookieCache$SetCookieCacheIterator.next(SetCookieCache.java:52)

After this change, we've had zero ConcurrentModificationExceptions and our crash-free sessions are up to over 99%.

Hashset is not thread-safe and the use of the synchronized keyword was still allowing the crash to happen in the iterator's next() method.

We are using this fork in our production app with no issues.

[tmm1]

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