The SQLite3 adapter name was referenced as
`ActiveRecord::ConnectionAdapters::Sqlite3Adapter`, but the
ActiveRecord class is spelled `SQLite3Adapter` (upper-case "SQL"). As
a result, `active_record/adapters/sqlite3_adapter.rb` wasn't actually
including the methods from `adapters/sqlite3_adapter.rb` into the AR
adapter class.
This adds a test for proper inheritance and fixes the spelling of
`SQLite3Adapter`, unifying all references to the ActiveRecord version.