[FIXED JENKINS-18534] equals()/hashCode() for parameter value

The default implementation considers parameters with the same name
to be identical, resulting in queue items being collapsed.

With this change, different values for a parameter will result in
different builds.
[JENKINS-18534] Added test contributed by @christ66
Merge pull request #87 from daniel-beck/JENKINS-18534

[FIXED JENKINS-18534] equals()/hashCode() for parameter value