Small problem with PostgreSQL table generation

CreateTable generates invalid SQL in Postgres after either DialectProvider.UseUnicode or DialectProvider.DefaultStringLength is changed, and the table contains string column.

First two commits are failing test + example of the generated SQL create statement. Third is a possible fix.

Kind of pointless to change "UseUnicode" in postgres provider, since the database determines whether strings are stored as unicode, or not unicode, but can happen, if you are targeting multiple databases...