CoGroupOisMapping Does Not Handle Multiple Entries For The Same Group With Different Validity

It is possible for an OIS backend to return multiple entries for the same group, with different validity dates. SalesforceSource can do this if, eg, there are multiple entries for the same committee or other object, each with a different validity window (say, one for 2016, a second for 2017, and a third for 2018).

In such a scenario, it appears that SalesforceSourceBackend::resultToGroups will return a list of all three memberships, but then CoOisGroupMapping::mapGroups will collapse down to one (the last one it sees). Either mapGroups should calculate the union of group validity (eg: 2016-2018) or the backend should not return multiple entries that map to the same group (the backend could do the union-ification if it so choses).