Fix saving a portfolio entry (fixes issue #748)

[e2jk]

When creating a new project from /+portfolio/editor/, the code was trying to get the pk of the project by getting it from request.POST['portfolio_entry__pk']. However for new projects there is no such element, so a MultiValueDictKeyError exception was launched, and since it was not catched it resulted in a 500 error.

My change uses the .get function of the request.POST object to retrieve that value, and gives it the value of undefined if it's not present. The next line already checks if pk == 'undefined':, so now there is no more exception being triggered, and the new project can be created.

This fixes https://openhatch.org/bugs/issue748

[paulproteus]

Emilien, this is a great patch; thank you for taking a look.

If possible, please send a license notice to the mailing list as per this link: http://openhatch.readthedocs.org/en/latest/contributor/handling_patches.html#permit-us-to-share-your-work

But since this is such a small fix, I'll merge it as-is!