use unmmodifiable map when possible

Author: lbalmaceda

src/main/java/com/auth0/exception/APIException.java

@@ -33,8 +33,8 @@ public APIException(String payload, int statusCode, Throwable cause) {
     public APIException(Map<String, Object> values, int statusCode) {
         super(createMessage(obtainExceptionMessage(values), statusCode));
         this.values = Collections.unmodifiableMap(values);
-        this.error = obtainExceptionError(values);
-        this.description = obtainExceptionMessage(values);
+        this.error = obtainExceptionError(this.values);
+        this.description = obtainExceptionMessage(this.values);
         this.statusCode = statusCode;
     }