Allowed "force enabling" of flat tables use for a product collection

[loekvangool]

Fixes #2914

[loekvangool]

Example on how to implement a fast flat table collection while in an area that until now blocks access to flat tables:

$productFlatHelper = Mage::helper('catalog/product_flat');
var_dump("<br>\$productFlatHelper->isFlatCollectionDisabled(): ".$productFlatHelper->isFlatCollectionDisabled()); // true if in cart
$productFlatHelper->enableFlatCollection(true); // call to the new method
var_dump("<br>\$productFlatHelper->isFlatCollectionDisabled(): ".$productFlatHelper->isFlatCollectionDisabled()); // false

// Get some products
$_products = Mage::getModel("catalog/product")->setPageSize(12)->setCurPage(1);
var_dump($collection->getSelect()->__toString()); // uses flat table

$productFlatHelper->resetFlatCollection();
var_dump("<br>\$productFlatHelper->isFlatCollectionDisabled(): ".$productFlatHelper->isFlatCollectionDisabled()); // back to true, not breaking anything

[elidrissidev]

Can you add a space after the if to make PHP-CS-Fixer check pass.

[fballiano]

I do not love this PR because of #2914 (comment) but, at the end of the day, it has to be forces by somebody in his/her own code so it's fine with me