#1086 identifier type in proxies

See #1086. This fix simply ensures that the column type for identifiers of proxies of STI/JTI are kept consistent with the DBAL types they are mapped to.

May be related/colliding with #1178

Ping @jaspernbrouwer 
A bit too verbose, but passing in `$targetClass`, `$srcColumn` and `$mapping` seemed overkill...