Account for field truncation when comparing new values to current values

Without comparing truncated values, a field which is truncated will
always be updated, only to be set to the same value as previously.  This
can occur particularly often when values are pulled from an external
source via RT::Authen::ExternalAuth or RT::Extension::LDAPImport.
Merge branch '4.0/avoid-updating-truncated-values' into 4.0-trunk