[DBAL-920] Use PDO::PGSQL_ATTR_DISABLE_PREPARES by default
Skip PDO::PGSQL_ATTR_DISABLE_PREPARES if already set
#714 - stripping tabs (spaces instead)
#714 - Adding a test verifying that attribute `PDO::PGSQL_ATTR_DISABLE_PREPARES` is set when using pgsql on PHP 5.6+
#714 - Cleaning up test case (dedicated private method for skipped cases)
#714 - prepares should be disabled by default, but skipped when defined and not set to `true`
#714 - aligning setting `PDO::PGSQL_ATTR_DISABLE_PREPARES` with test specification
#714 - adding missing `@group` annotations for reference
#714 - `db_type` may not be defined, and `db_name` may be invalid
#714 - ignoring PDOExceptions on PDO#getAttribute(PDO::PGSQL_ATTR_DISABLE_PREPARES) as per https://bugs.php.net/bug.php?id=68351
#714 - marking test as incomplete when affected by https://bugs.php.net/bug.php?id=68371
DBAL-920 - Merge branch 'hotfix/#714-disable-prepares-on-pgsql-for-php-5.6'

Close #714