[patch] SchemaDumper works bad with table_prefix

When setting ActiveRecord::Base.table_name_prefix or table_name_suffix, Rails will dump all tables found in the database (except schema_migrations). This is a little dangerous when you do ie. rake db:reset. IMHO, the convention should be to dump only tables with the set prefix and suffix.

Of course, there is ActiveRecord::SchemaDumper.ignore_tables, but it lets you only define a blacklist, which is cumbersome on prefixes. A prefix like 'abc' would need

ActiveRecord::SchemaDumper.ignore_tables = [ /^(?!abc)/ ]

which is advanced regex magic and maybe too complicated for beginners.

Therefore I propose adding ActiveRecord::SchemaDumper.dump_only_with_prefix and -suffix, where you can turn off the new default behaviour.

This also fixes a small bug, where the table name 'schema_migrations' is hard coded instead of being used through ActiveRecord::Migrator.schema_migrations_table_name.