Change default subjectDnRegex pattern in SubjectDnX509PrincipalExtractor to match CN at end of DN

[michaelpigg]

Changes subjectDnRegex default pattern from "CN=(.?)," to "CN=(.?)(?:,.)$" to match a CN occurring anywhere in the subject DN field. Added unit test to show that this works and does not break existing unit tests.

[rwinch]

Thank you for your submission. Can you please update your pull request to conform to the recently created Contributor Guidelines? I realize that the guidelines were created after the pull request, but in order for me to accept the pull request the guidelines should be followed. For example, single commit per JIRA, associated to a JIRA (I know there is a JIRA but the commit comment should have the JIRA ID as described in the guidelines), create a topic branch that is rebased (not merged), etc.

[rwinch]

I have modified the commits myself and pushed them to master. I also modified the regular expression slightly to more eagerly terminate. I used your test cases, so left @michaelpigg as the author on them but updated the commit message to conform to the contributor guidelines. Once again thank you for your submission.