Change default for JSON.Feature.USE_FIELDS to true (from false) in 2.10

[cowtowncoder]

Since JSON.Feature.USE_FIELDS was added as a new thing, it defaulted to false.
That seemed like a good idea but since it seems to lead to problem as new users do not realize that such an option is available. Enabling it would make default behavior more similar to main Jackson, and so although usually we'd prefer not changing defaults this seems like one exception.
So, let's change setting for 2.10.