Hotfix #376: Accessing constraint metadata within the Oracle adapter

[panychek]

Fixes #376

The OracleMetadata::loadConstraintData method simply does not work:

$metadata = \Zend\Db\Metadata\Source\Factory::createSourceFromAdapter($adapter);
$constraints = $metadata->getConstraints($table, $schema); // always an empty array

The existing test couldn't catch it because there is no real table involved:

zend-db/test/unit/Metadata/Source/OracleMetadataTest.php

Lines 48 to 49 in 4c68f2c

	$constraints = $this->metadata->getConstraints(null, 'main');
	self::assertCount(0, $constraints);

[michalbundyra]

@panychek Thanks for your contribution. We need unit test for that change.

[panychek]

I modified the existing one, so please have a look.
BTW, there are no tests for this behavior in either of the adapters.

[michalbundyra]

Thanks, @panychek!