Silently truncating the data to fit into a column is a bad idea, just don't do
this.

Also add a unit test verifying that an exception is indeed thrown if the value
being inserted doesn't fit in the column, except for SQLite where anything
fits and MySQL where it doesn't but just gets silently truncated by the
database by default.

Finally add a helper on_after_ddl() method to the test context class to allow
creating tables in the common test code instead of having to always do it in
RDBMS-specific way, even when the DDL uses completely standard SQL and the
only difference is that Firebird needs a commit after executing it.

Closes #51.