This will solve the issue that abort the connection transaction when we
skip the tests.

This implements the support to encode/decode JSON
data to/from database and creating columns of type
JSON using a native type [1] supported by PostgreSQL
from version 9.2.

[1] http://www.postgresql.org/docs/9.2/static/datatype-json.html

Didn't fail the test because adapter#query happens to
not call raw connection's #query, but don't want to count
on that and have a fragile test.

Full test requiring manual intervention was fine, but
w/ simulated disconnect, assertion was backward & still
passing. Was several kinds of wrong.

This patch addresses the difficulty of retrieving datetime fields. By default, the database holds a higher precision than the time as a String.

This issue is discussed at length at the following links:
- [#3519](https://github.com/rails/rails/issues/3519)
- [#3520](https://github.com/rails/rails/issues/3520)

Also, kudos to @mattscilipoti

escape_hstore uses quotation marks around keys and values only if it
seems necessary. However, it currently breaks in the presence of some
non-ASCII characters. Instead of trying to guess exactly which
characters are safe, it seems better to always use quotes.

Restored ability to identify ID and Sequence from tables relying on a nonmatching sequence default value for PK.

The `add_index` method now supports a `where` option that receives a
string with the partial index criteria.

    add_index(:accounts, :code, :where => "active")

    Generates

    CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active