refactor: create lowerCaseChar var

Author: vbetsch

src/main/java/io/github/vbetsch/codecracker/CodeCracker.java

@@ -22,8 +22,9 @@ public String decryptString(String string) {
     public String encryptString(String string) {
         StringBuilder stringBuilder = new StringBuilder();
         for (char character : string.toCharArray()) {
-            if (!dictionary.containsDecryptionValue(Character.toLowerCase(character))) continue;
-            stringBuilder.append(encryptLetter(Character.toLowerCase(character)));
+            char lowerCaseChar = Character.toLowerCase(character);
+            if (!dictionary.containsDecryptionValue(lowerCaseChar)) continue;
+            stringBuilder.append(encryptLetter(lowerCaseChar));
         }
         return stringBuilder.toString();
     }