Fix intermittent PDT issues

[djw]

Fixes issue #239.

Rather than use the same form for both the PDT callback and the postback, split these apart. We'll take a minimal set of parameters from the callback, and fill in rest using the postback endpoint.

I've included a test which includes the full set of query parameters I'm seeing on most (but not all) PDT callback requests. Note payment_date is in ISO format in these requests, and notify_version is a string. With this PR both these parameters will be ignored.

[spookylukey]

Sorry it has taken me so long to look at this, see my question below.

[spookylukey]

If I've read it correctly, the only piece of information that we use is tx (looking at PayPalPDT._postback(). Do you agree, if so, should we make that the only field? Are all the other fields set from the postback call? My reading of the PayPal docs suggests that it is actually just the transaction ID we are looking for at this point.

(Please forgive my ignorance, I've only ever personally used IPN)

It would be great, either way, if we could have a reason for the specific set of fields included here, and a comment on PayPalPDTCallbackForm that explained its purpose.

[djw]

I can't remember my logic, but I assume I chose these four to match the PDT-specific fields here:

django-paypal/paypal/standard/pdt/models.py

Lines 33 to 37 in 9c75afe

	amt = models.DecimalField(max_digits=64, decimal_places=2, default=0, blank=True, null=True)
	cm = models.CharField(max_length=255, blank=True)
	sig = models.CharField(max_length=255, blank=True)
	tx = models.CharField(max_length=255, blank=True)
	st = models.CharField(max_length=32, blank=True)

I suspect I omitted sig because it doesn't appear in either the old-style or new-style query parameters.

As you say, tx is the only field used explicitly, but the others are saved to the instance, and I wanted to avoid introducing any changes in behaviour in this PR.

Having said all that, I'm not convinced that PayPalPDT needs these custom fields at all — all the interesting information comes from the postback and is saved to fields defined in PayPalStandardBase.