Add mapper feature to allow explicitly named properties to be renamed with PropertyNamingStrategy

[dharaburda]

The fix for [#428] made a change to prevent explicitly named properties (ones that have JsonProperty("explicitName") for example) from being renamed by a configured PropertyNamingStrategy, however sometimes this behavior may be desirable.

This simple change adds a mapper feature (disabled by default of course) that allows this behavior.

[dharaburda]

If accepted, it would be great if this could make into a 2.6.x release (assuming 2.7 is further out?) I'm also happy to update any docs if someone points me to the right place.

[cowtowncoder]

Looks good to me. Unfortunately it can not be added in 2.6, since it is addition to API, which is something patch releases should not do (because it'd mean that patches are not fully API compatible etc -- but this is just a rule we follow), but makes sense for 2.7.

So, I'd be happy to merge this contribution.

One practical thing we will need now is the Contributor License Agreement (from https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf).
It is needed before the first contribution, but not needed afterwards.
Most developers print it, fill, sign, scan and email to info at fasterxml dot com. If you could do that it would be great; apologies for the hassle, but this is the only part of formal process we have. :)

Looking forward to merging this feature!

[dharaburda]

No problem regarding 2.7. I have e-mailed the signed agreement. Thanks!

[cowtowncoder]

Looks good to me, CLA received, merging.

[cowtowncoder]

Beautiful. It also gave me an idea of a follow-up feature to perhaps implement; will file the issue next, link to this one.

[cowtowncoder]

So, #926 is for adding property in @JsonNaming to override this global setting on per-type basis.
Not sure how easy that'd be to implement, but seems like something that would be useful both for enforcing renaming on just some types, and for possibly preventing that (if default was to allow).