Force principalId to be a string.

Added a defensive check for undefined to avoid problems
when converting to a string.
Adjust id handling to deal with 0 and null
Make sure GET /:id/exists returns 200 {exists: true|false}
https://github.com/strongloop/loopback/issues/679
Merge tag 'v2.6.0'

2.6.0
Merge pull request #681 from strongloop/feature/fix-issue-679

Make sure GET /:id/exists returns 200 {exists: true|false}
Merge pull request #680 from arlaneenalra/feature/fix-676

Force principalId to be a string in Role.getRoles Fix #676
Add realm support
Merge pull request #660 from strongloop/feature/add-realm-support

Add realm support
User: custom email headers in verify
Merge pull request #616 from jpizarrom/master

added email custom headers in user verify
Bump version
Merge branch 'release/2.7.0' into production