When changing a field's NULL constraint from true to false and

setting the initial data, database evolver backends were assuming the

data being set could be injected directly into the query in the

database. This was often valid, but fell down for more complex types

(like dates/datetimes).
We now accept setting a string directly (for compatibility), but

anything else gets turned into a %s with the value going into the SQL

parameter list.
This ensures that the database layer can properly pass in the data type

in a way that the backend will like.

Unit tests pass for all versions of Django on all database types.