This reverts commit c321b309.

Conflicts:
	activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
	activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

Reason: failing test

  1) Error:
test_valid_column(ActiveRecord::ConnectionAdapters::SQLite3AdapterTest):
NoMethodError: undefined method `column' for
    test/cases/adapters/sqlite3/sqlite3_adapter_test.rb:29:in
`test_valid_column'

* add/cleanup tests

* tidy test code and fix my typo 

* table_exists? scoped by schema search path unless schema is explicitly named. Added tests and doc to clarify the behaviour
* extract_schema_and_table tests and implementation extended to cover all cases
* primary_key does not ignore schema information
* add current_schema and schema_exists? methods
* more robust table referencing in insert_sql and sql_for_insert methods

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.