Add some tests to enumerate how extensions should be stored in the schema output
improve tests to check for existence of extensions method, and skip testing dumped extensions if they are unsupported by the database
add ActiveRecord::AbstractAdapter#extensions and ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#extensions to allow dumping of enabled extensions to schema.rb, add ActiveRecord::SchemaDumper#extensions to dump extensions to schema.rb
Merge pull request #9203 from jaggederest/dumping_pgsql_extensions

Adding database extension support to schema.rb