[merb_datamapper] Perform IM wrapping at the rack level
[merb_datamapper] Perform IM wrapping at the rack level

This fixes an issue where before/after filters wouldn't
make use of the identity map because previously only
Merb::Controller#_call_action was wrapped.

merb/merb,merb/merb,wycats/merb,wycats/merb,wycats/merb,wycats/merb,wycats/merb,merb/merb