filter/date: Reject invalid UNIX timestamp

When using the date filter with UNIX or UNIX_MS format, if an invalid value is given like a not-resolved sprintf %{mytimestamp} , it is evaluated to zero (thanks String#to_i) and thus reset @timestamp to EPOCH time.

Root cause of #1236 and also mentionned in LOGSTASH-1597

Bugfix + spec!
@wiibaa Can you make this non capturing group? `/^\d+(?:\.\d+)?$/` Will give it some performance gain