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

Author: jizhuozhi

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

@@ -46,7 +46,7 @@ public enum ProtocolVersion {
     HTTP_2("HTTP/2.0"),
     MOCK;
 
-    String protocolVersion;
+    final String protocolVersion;
 
     ProtocolVersion() {
       protocolVersion = name();

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

@@ -331,7 +331,7 @@ public enum EncodingOptions {
     REQUIRED(true),
     NOT_REQUIRED(false);
 
-    private boolean shouldEncode;
+    private final boolean shouldEncode;
 
     EncodingOptions(boolean shouldEncode) {
       this.shouldEncode = shouldEncode;