Ensure that XML module uses registered `XmlMapper` (not just `ObjectMapper`)

[cowtowncoder]

One possible problem with different backend format JAX-RS providers is that one can only register a single ObjectMapper via JAX-RS. By default it is reasonable to assume that it is probably right for formats other than JSON.

There are two basic approaches to resolve this problem:

1. For formats that require custom ObjectMapper, such as XML (which requires XmlMapper), look up that mapper instead of default one.
2. For formats that may use default ObjectMapper, verify that the backend format (as identified by ObjectMapper.getFactory()), and if not matching, construct a variation with proper factory instance.

Obviously additional tests are needed, both to verify default behavior, as well as ability to (re)configure mappers.

[cowtowncoder]

Verified -- this is indeed the case now.