Symbology legend classification items can get out of sync with classification field

[springmeyer]

Using current symbology (not sym-ng) and a postgis dataset (openstreetmap) it is possible to:

1) choose unique value symbology

2) choose a classification field of text and classify features

3) then still within the symbology interface change the classification field to a int4 type

4) then leave the symbology interface without again hitting 'classify'

I know this is a bit of a fringe thing, and I'm not sure out the right solution, but I noticed this because somehow I managed to do this and it breaks Quantumnik (and I assume other plugins that query the classification field type and value), because QGIS reports int4 (Qvariant.Int) type but returns text-based feature values.