Fix #643: Add ToXmlGenerator.Feature or allowing XML Schema/JAXB compatible Infinity representation

[ahcodedthat]

This PR adds a ToXmlGenerator.Feature to use XML Schema-compatible representation for floating-point infinity.

Fixes #643.

[cowtowncoder]

Pretty good but maybe add WRITE_ prefix to go along with existing WRITE_NULLS_AS_XSI_NIL?

[cowtowncoder]

Looks good; added some minor suggestions to change.

[cowtowncoder]

@ahcodedthat Excellent, looks good, I like the implementation.

One process thing before I can merge this: if we do not yet have CLA from you (one is enough for all contributions so this is needed just once), we'd need one from:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

(there's also corporate CLA as an alterative if preferred).

It is usually easiest to print, fill & sign, scan/photo, email to cla at fasterxml dot com.

Once I have it, can proceed with merge and this should nicely get into 2.17.0 as one of the (if not the :)) last feature to be included.

Looking forward to merging this nice addition; thank you again for submitting it!

[cowtowncoder]

Hoping to get CLA resolved by tomorrow as I will need to proceed with 2.17.0 release.
Would not matter otherwise -- often things can go in patches -- but this is API change (addition) so needs to go in a minor release as per SemVer.

[ahcodedthat]

Sorry for the delay! I just sent the filled-out CLA.

[cowtowncoder]

Thank you @ahcodedthat for sending CLA -- I was counting on it getting done soon so it made it in 2.17! :)