fixed incomplete UnitOfWork::clear()

UnitOfWork's clear() method (called by DocumentManager's clear() method) does not reset UnitOfWork's `$persisters` attribute, which leads to an empty/zombie document being inserted in the repository if a (valid) object is persisted/flushed after a previous flush fails.

Here's some sample code showing the problem.

	<?php

	require_once 'bootstrap-odm.php';
	$dm = DocumentManager::create(new Connection('mongodb://sandbox:sandbox@localhost/sandbox'), $config);

	use Doctrine\ODM\MongoDB\Mapping\Annotations as ODM ;

	/** @ODM\Document(db="sandbox",collection="duptest") */
	class Foo {

		/** @ODM\Id */
		protected $id;

		/** @ODM\Field(type="string") */
		protected $uniquefield;

		public function setUniquefield($string) {
			$this->uniquefield = $string;
		}
		
		public function getId() {
			return $this->id ;
		}

	}

	// clean up first
	$foo = $dm->getRepository('Foo')->findOneByUniquefield('bar');
	if (!is_null($foo)) {
		echo 'Removed previously existing document' . "\n";
		$dm->remove($foo) ;
		$dm->flush();
		$dm->clear();
	}

	// repository should be empty
	dump_repository($dm) ;

	// create, persist and flush initial object
	$foo1 = new Foo();
	$foo1->setUniquefield('bar') ;
	$dm->persist($foo1);
	$dm->flush();
	$dm->clear();

	// create and persist second object with dup'd unique field
	$foo2 = new Foo();
	$foo2->setUniquefield('bar') ;
	$dm->persist($foo2);

	// flush fails
	try {
		$dm->flush();
	}
	catch (MongoCursorException $e) {
		echo 'Expected exception as unique field is marked as unique' . "\n";
	}

	$dm->clear();

	// remove initial object
	$foo = $dm->getRepository('Foo')->findOneByUniquefield('bar');
	$dm->remove($foo) ;
	$dm->flush();
	$dm->clear();

	// an empty flush won't be a problem
	$dm->flush();

	// but now let's create a third object
	$foo3 = new Foo();
	$foo3->setUniquefield('foo') ; // could be anything else
	$dm->persist($foo3);
	$dm->flush();
	$dm->clear();

	/* repository should contain 1 object, actually contains 2, one of which is
	 * empty (only the id is populated) */
	dump_repository($dm) ;

	/* Running this code again will lead to:
	MongoCursorException: localhost:27017: E11000 duplicate key error index: sandbox
	.duptest.$uniquefield_1  dup key: { : null } in 
	…\doctrine-mongodb\lib\Doctrine\MongoDB\Collection.php on line 223
	 */

	function dump_repository(Doctrine\ODM\MongoDB\DocumentManager $dm) {
		$foos = $dm->createQueryBuilder('Foo')->getQuery()->execute();
		echo 'Repository  contains ' . count($foos) . ' object(s):' . "\n" ;
		foreach ($foos as $foo) {
			var_dump($foo) ;
		}
	}
	?>

This code yields (notice the first empty/zombie object Foo#46):

	Repository  contains 0 object(s):
	Expected exception as unique field is marked as unique
	Repository  contains 2 object(s):
	class Foo#46 (2) {
	  protected $id =>
	  string(24) "518fac5d101e3ce412000002"
	  protected $uniquefield =>
	  NULL
	}
	class Foo#55 (2) {
	  protected $id =>
	  string(24) "518fac5d101e3ce412000003"
	  protected $uniquefield =>
	  string(14) "foo"
	}

Expected behaviour (after fix) is:

	Repository  contains 0 object(s):
	Expected exception as unique field is marked as unique
	Repository  contains 1 object(s):
	class Foo#46 (2) {
	  protected $id =>
	  string(24) "518fbc13101e3ca414000002"
	  protected $uniquefield =>
	  string(3) "foo"
	}