Fixed issue #3392 clearInstance() method in Mage_Catalog_Model_Product.

[kiatng]

Description (*)

As explained in issue #3392, clearInstance() kept the previous product type. The reason is because the properties $_typeInstance and $_typeInstanceSingleton were not reset. I reviewed all other properties in Mage_Catalog_Model_Product and its parent Mage_Catalog_Model_Abstract and reset them.

Fixed Issues (if relevant)

1. Issue In catalog/product model the clearInstance function doesn't clear typeInstanceSingleton #3392

Manual testing scenarios (*)

Test script:
In my test, product 1 is type downloadable, product 2 is type virtual.

    $p = Mage::getModel('catalog/product')->load(1);
    $r['downloadable'] = get_class($p->getTypeInstance(true));
    $p->clearInstance()->load(2);
    $r['virtual'] = get_class($p->getTypeInstance(true));

Expected result (with this PR):

 array(2) {
  ["downloadable"] => string(36) "Mage_Downloadable_Model_Product_Type"
  ["virtual"] => string(39) "Mage_Catalog_Model_Product_Type_Virtual"
}

But actual result (without this PR):

 array(2) {
  ["downloadable"] => string(36) "Mage_Downloadable_Model_Product_Type"
  ["virtual"] => string(36) "Mage_Downloadable_Model_Product_Type"
}

[addison74]

3 issues with PHPStan.

[fballiano]

checked the defaults and they look ok