fix: Currency Exchange for_selling and for_buying on the same day

[toofun666]

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In this fix:

• You can now add a separate exchange_rate in date for_selling and for_buying. You could not before because the unique field name was only calculated to allow a single name for a date
• tests did not account for for_selling and for_buying fields and thier uniqueness

[toofun666]

Dears,
this update will help the vanilla systems to function better in Currency Exchange. The test has also been updated to cover the full scenario where for_selling and for_buying are required fields but were not covered in the test.

[toofun666]

Let us get this going to put the design back into place with proper testing on Currency Exchange.
The problem was:

• for_selling or for_buying was a must
• the naming was based on parameters excluding these fields
• thus you could not enter a buying and selling currency exchange rate for the same day because of unique naming construction