MONTHDAY must not require a leading zero for day-of-month < 10

[purbon]

The MONTHDAY as is used by, among others, the RFC 2822 date stamp, must not require a leading zero for day-of-month < 10.

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From: https://logstash.jira.com/browse/LOGSTASH-2199 and elastic/logstash#1353

Relates to #5 (PR closed because of stale CLA request)