SEC-3175: Migrate to AssertJ

[spring-projects-issues]

Rob Winch (Migrated from SEC-3175) said:

We should migrate fest assert, JUnit, and Hamcrest assertions to AssertJ

[spring-projects-issues]

Rob Winch said:

This is started in a branch https://github.com/rwinch/spring-security/tree/SEC-3175-assertj

[TYsewyn]

@rwinch I'll pick this up further, if it's ok?

[rwinch]

@TYsewyn Thanks for the offer! I'm real excited how active you have been :) I think this one will be taken care of in a PR #248 I'm just struggling to get all the PRs I've been getting merged.

[rwinch]

Fixed via 71d4ce9