Vulnerability.vulnerability_definition does not load the correct vuln def

[gschneider-r7]

Reported on community here: https://community.rapid7.com/thread/7935

The Nexpose API itself is behaving correctly, but the gem is somehow only applying the last vulnerability_definition in an array of vulnerabilities, and then caching it (I think) such that any other vulnerability objects calling vulnerability_definition do not send a request (and thus return nil).