Change SpEL equality operators to use .equals

Prior to this commit the SpEL operators `==` and `!=` were using the
Java `==` comparison operator as part of their equality checking. It is
more flexible to use the equals() method on Object.

Under this commit the change to .equals() has been made and the equality
checking code has been pushed into a common method in the Operator
superclass. This commit also makes some tweaks to the other operator
classes - the Float case was missing from OpGT.

Issue: SPR-9194
Merge pull request #406 from aclement/fix-SPR-9194

* fix-SPR-9194:
  Change SpEL equality operators to use .equals