[h2] support empty insert statement (actually used by AR 4.0)
adjust tests to not use empty insert statement (on 4.0) due DbType.create!
empty_insert_statement_value for Derby and HSQLDB won't work
4.0 compatibility in HSQLDB/H2 schema dump tests
pend select rows with AREL (on AR-4.0) on has_many test
override #sturcture_dump so it won't silently return while doing nothing
JdbcConnection: over-load #table_exists? + use toString with name args

cause otherwise when symbols are passed they fail due missing #to_str

- also support #indexes(table_name, name) without schema name
re-build adapter jar