feat: improve Rails zones mapping behavior

[mattjohnsonpint]

When mapping to a Rails zone, if there's no Rails short name, we should not give up! Rails supports IANA names directly, and also supports fixed offsets.

This PR changes the *ToRails behavior such that:

• If the mapping is a fixed offset, then return the fixed offset such as +06:00 or -03:00
• If there's a valid Rails short name to use, then continue to use it.
• Otherwise, return the IANA name

One edge case - retain the Dateline Standard Time Windows to International Date Line West Rails mapping. (but use Etc/GMT+12 for IANA)