fixes #803 PatternLayoutBase custom converter classloading in OSGi

[enapps-enorman]

This is the proposed changes to resolve issue #803

This is an alternate way to declare the PatternLayoutBase defaultConverterMap and instanceConverterMap values in a way that works with the OSGi classloading restrictions.

These changes should preserve backward compatibility with code that defines the converter map using the original technique

I also updated the tests in an attempt to confirm it works as expected.

Please review and merge if you agree with the proposed changes.

[enapps-enorman]

Hi @ceki

We would appreciate a review of this proposal if you (or someone else) has the time. Knowing if this proposal will be accepted or not will help us decide how to move forward.

Thanks