fix prefetch related with UUID fields

[berndtj]

Because the foreign key is stored as a string (char field) and the uuid field is returned as a UUID (StringUUID) object, prefetch related does not work.

This patch makes StringUUID dict keys equivelent to string dict keys.

[berndtj]

This fixes the prefetch related issue and does not break existing code since the .hex method can still be called on the StringUUID object.

[luto]

Looks good to me. Any concerns, @dcramer?

[sshishov]

The fix is not working when you are using hyphen = True in your model. In this case in the dictionary it will be UUID with hyphen and checked against hex format what is incorrect. IMHO you should __hash__ and __eq__ do against hex values always to be determenistic