Change SpEL equality operators to use .equals

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