Make EnumTypeAdapter friendly with obfuscation

When enum value was obfuscated by proguard, EnumTypeAdapter raise NoSuchFieldException even if apply SerializedName annotation.
Because EnumTypeAdapter cannot find obfuscated enum constant field with its name.

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);
       }
     }