Extract floor entry recuperation

tcr-roman-numerals/src/main/java/fr/ippon/kata/numeral/Numerals.java

@@ -10,7 +10,11 @@ public class Numerals {
   private static final NavigableMap<Integer, String> CONVERSIONS = buildConversions();
 
   public static String toRoman(int arabic) {
-    return Optional.ofNullable(CONVERSIONS.floorEntry(arabic)).map(romanRepresentation(arabic)).orElse("");
+    return highestKnownConversion(arabic).map(romanRepresentation(arabic)).orElse("");
+  }
+
+  private static Optional<Entry<Integer, String>> highestKnownConversion(int arabic) {
+    return Optional.ofNullable(CONVERSIONS.floorEntry(arabic));
   }
 
   private static Function<Entry<Integer, String>, String> romanRepresentation(int arabic) {