[JEP-200] Save charset names, not actual Charset objects

[jglick]

See jenkinsci/jenkins#3120. Note that

• DescriptionSetterWrapperDescriptor was already doing the right thing.
• The current getter is not right—should return .name(), not .displayName().
• This patch does not address old saved settings, which would presumably block the wrapper from loading. You would need to resave the project before updating Jenkins. Fixing that would be hard—would need a custom XStream Converter, I think.

@reviewbybees

[ghost]

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[oleg-nenashev]

🐝

[jglick]

@reviewbybees done

[oleg-nenashev]

Created a request to get temporary ownership of the plugin: https://groups.google.com/forum/#!topic/jenkinsci-dev/scG6cjRCqxY