currency: Fix edge case in NewCode where all string characters are digits (#1593)

* add fixes and test

* glorious: nits, privatise upper case field, add item field for case sensitivity for format checks, rm dead code.

* fix potential panic

* gk/glorious: nits

* gk: nits and other things

* improve commentary lol

* glorious+gk: nits and improvements (with no sillyness this time)

* Update currency/pairs.go

Co-authored-by: Adrian Gallagher <adrian.gallagher@thrasher.io>

* thrasher: nitssssss

* linter: fix

* bye bye silly boy

---------

Co-authored-by: Ryan O'Hara-Reid <ryan.oharareid@thrasher.io>
Co-authored-by: Adrian Gallagher <adrian.gallagher@thrasher.io>

exchanges/account/account.go

@@ -112,7 +112,7 @@ func GetHoldings(exch string, creds *Credentials, assetType asset.Item) (Holding
 		}
 		assetHoldings.m.Lock()
 		currencyBalances = append(currencyBalances, Balance{
-			Currency:               currency.Code{Item: mapKey.Currency, UpperCase: true},
+			Currency:               mapKey.Currency.Currency().Upper(),
 			Total:                  assetHoldings.total,
 			Hold:                   assetHoldings.hold,
 			Free:                   assetHoldings.free,

exchanges/bitfinex/bitfinex_wrapper.go

@@ -1177,9 +1177,14 @@ func (b *Bitfinex) GetHistoricCandlesExtended(ctx context.Context, pair currency
 }
 
 func (b *Bitfinex) fixCasing(in currency.Pair, a asset.Item) (string, error) {
-	if in.IsEmpty() || in.Base.IsEmpty() {
+	if in.Base.IsEmpty() {
 		return "", currency.ErrCurrencyPairEmpty
 	}
+
+	// Convert input to lowercase to ensure consistent formatting.
+	// Required for currencies that start with T or F eg tTNBUSD
+	in = in.Lower()
+
 	var checkString [2]byte
 	if a == asset.Spot || a == asset.Margin {
 		checkString[0] = 't'