Changes to Get rates for a base currency effective from a specific time

changed the pattern of the path request parameter base from ^[A-Z]{3}$ to ^[A-Z]{3,4}$

the results/items/base response's property pattern was changed from ^[A-Z]{3}$ to ^[A-Z]{3,4}$ for the status 200

the results/items/target response's property pattern was changed from ^[A-Z]{3}$ to ^[A-Z]{3,4}$ for the status 200