Fixed #13844 -- Avoid converting unknown db values to float

This patch removes an unconditional float(value) conversion from db
backend default convert_values() method. This can cause problems when
aggregating over character fields for example. In addition, Oracle
and SQLite already return the bare value from their convert_values().

In the long term the converting should be done by fields, and the
fields should then call database backend specific converters when
needed. The current setup is inflexible for 3rd party fields.

Thanks to Merlijn van Deen for the original patch.
(cherry picked from commit 59a655988ee95e4b4e4646cebea88b620d8fcdd2)
Refix #13844 -- Made FloatField aggregates work on Python 2.6 + Postgres

Fixed a regression introduced in 59a655988ee95e4b4e4646cebea88b620d8fcdd2.
(cherry picked from commit 1930b899bd69fb084856257598ab5ddd8ee4e016)