Proposed fix for Issue #81 (deprecated constructor always throws an exception)

[JCgH4164838Gh792C124B5]

Proposed fix for Issue #81:

• Drops OgnlContext(Map values) constructor that always fails with an
  Exception.
• Additional comments for remaining two constructors.
• Changed constructor to ensure _values Map can no longer be null.
• Added comments to setValues(Map value) and getValues() for clarity.
• Added @OverRide annotations to Map interface methods.

[JCgH4164838Gh792C124B5]

Hello OGNL Team.

This PR should address the concern raised in the issue with a little low-risk cleanup added as well.

Other than dropping the deprecated constructor, this PR also ensures the context's values Map cannot be initialized to null (given the usage in the rest of the class, allowing a null values Map does not seem to make sense). That could impact someone who intentionally set the values to null, but preventing this in 3.2.x seems worth that small risk (and the JavaDoc comments indicate null parameters will be set to defaults - except for MemberAccess which cannot be null).

Let me know what you think.

[lukaszlenart]

LGTM 👍

[lukaszlenart]

Struts build passed with a SNAPSHOT version, I will wait a bit before pushing a new release