Refactor outbound message encoder selection into a distinct method to:

Refactor outbound message encoder selection into a distinct method to:
1) consolidate logic and avoid duplication of code
2) check and avoid NPE's if endpoint or binding is null due to misconfiguration
3) allow subclasses to override encoder selection mechanism (re: uPortal work)

git-svn-id: https://subversion.switch.ch/svn/shibboleth/java-idp/branches/REL_2@2865 ab3bd59b-922f-494d-bb5f-6f0a3c29deca