SerializedName annotation workaround for obfuscated enums by ProGuard

SerializedName annotation cannot be applied on enums when obfuscated by
proguard.
Because EnumTypeAdapter cannot find obfuscated enum constant field
with its name.
So, EnumTypeAdapter should load annotation from DeclaredFields, not EnumConstants

gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java

@@ -17,6 +17,7 @@
 package com.google.gson.internal.bind;
 
 import java.io.IOException;
+import java.lang.reflect.Field;
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.net.InetAddress;
@@ -776,9 +777,14 @@ private static final class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapte
 
     public EnumTypeAdapter(Class<T> classOfT) {
       try {
-        for (T constant : classOfT.getEnumConstants()) {
+        for (Field field : classOfT.getDeclaredFields()) {
+          if (!field.isEnumConstant()) {
+            continue;
+          }
+          field.setAccessible(true);
+          T constant = (T)(field.get(null));
           String name = constant.name();
-          SerializedName annotation = classOfT.getField(name).getAnnotation(SerializedName.class);
+          SerializedName annotation = field.getAnnotation(SerializedName.class);
           if (annotation != null) {
             name = annotation.value();
             for (String alternate : annotation.alternate()) {
@@ -788,7 +794,11 @@ public EnumTypeAdapter(Class<T> classOfT) {
           nameToConstant.put(name, constant);
           constantToName.put(constant, name);
         }
-      } catch (NoSuchFieldException e) {
+      } catch (IllegalAccessException e) {
+        throw new AssertionError(e);
+      } catch (NullPointerException e) {
+        throw new AssertionError(e);
+      } catch (ExceptionInInitializerError e) {
         throw new AssertionError(e);
       }
     }