Fix usage of currencyCode property that isn’t available in older SDKs

Author: Ricardo Santos

Classes/MPNumericTextField.m

@@ -64,7 +64,7 @@ - (id)init {
 
 - (void) setDefaults {
   self.locale = [NSLocale currentLocale];
-  self.currencyCode = self.locale.currencyCode;
+  self.currencyCode = [self.locale objectForKey:NSLocaleCurrencyCode];
   self.keyboardType = UIKeyboardTypeDecimalPad;
   self.type = MPNumericTextFieldDecimal;
   self.delegate = self.numericDelegate;