Allow repeated calls to SimpleObjectIdResolver.bindItem() for same mapping

[cowtowncoder]

(note: work-around for FasterXML/jackson-dataformats-text#231)

Looks like there are cases where it would be useful to allow multiple calls to adding same mapping for "object-id -> instance" mapping. While ideally calls should not be repeated, it seems that guaranteeing this gets more complicated with native object ids; and in case of YAML with need to location-shift Object Id as well.
Since checking for duplicates is already made this should be easy enough to do.