Added test and PurchaseRequestRepo methods.

In the spec file, added a test for changing status of a purchase request.
Added method check_availability for PurchaseRequestRepo.
check_availability gets price from Breed class and quantity from Puppy Class and then checks if there is a suitable puppy to fulfill this purchase request
Also added method change_status to PurchaseRequestRepo
Change_status takes in request number and the status you want to change to and changes the status for the request.
Both methods have been tested and passed the test succesfully.