[Jaxrs-cxf] Unittests: uncomment api invocations, add Multipart snippet

[jfiala]

PR checklist

• [x] Read the contribution guildelines.
• [x] Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• [x] Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

see #4513

• Uncomment api invocations in unittests
• Also add processing for void to AbstractJavaCodegen.java so it is available to all languages.
• Add snippet to deal with CXF Attachments

Sidenote: the currently used PetApi.uploadFile with the FormParam additionalMetadata doesn't work with CXF as it only supports one Attachment parameter. If the parameter additionalMetadata is removed, the unittest works.

Re-created PR from PR #4823 due to broken compare view after rebase.

[wing328]

@jfiala there are many errors reported by the CI: https://s3.amazonaws.com/archive.travis-ci.org/jobs/210343309/log.txt

Please take a look when you've time.