Fix typo in emitter attr name

[nitzmahone]

This change was originally directly merged to a pending 6.1 release branch, but in its current form, it's technically a breaking change, since the renamed attribute is "public" on the Emitter mixin (and thus on all Dumpers that use that mixin). The fix is fine, but should at least temporarily preserve a compatibility property for the old name that fires DeprecationWarning for some time to preserve backward compatibility for existing consumers.

[nitzmahone]

(added shim property for backwards compatibility with DeprecationWarning on use)