fix: field of enum must be final (#1577)

core/src/main/java/feign/Request.java

@@ -36,7 +36,7 @@ public final class Request implements Serializable {
   public enum ProtocolVersion {
     HTTP_1_0("HTTP/1.0"), HTTP_1_1("HTTP/1.1"), HTTP_2("HTTP/2.0"), MOCK;
 
-    String protocolVersion;
+    final String protocolVersion;
 
     ProtocolVersion() {
       protocolVersion = name();

core/src/main/java/feign/template/Template.java

@@ -329,7 +329,7 @@ public class Template {
   public enum EncodingOptions {
     REQUIRED(true), NOT_REQUIRED(false);
 
-    private boolean shouldEncode;
+    private final boolean shouldEncode;
 
     EncodingOptions(boolean shouldEncode) {
       this.shouldEncode = shouldEncode;