Drop 500 from the retriable errorcode list (#576)

Author: chong-shao

src/main/java/com/google/firebase/internal/ApiClientUtils.java

@@ -34,7 +34,7 @@ public class ApiClientUtils {
 
   static final RetryConfig DEFAULT_RETRY_CONFIG = RetryConfig.builder()
       .setMaxRetries(4)
-      .setRetryStatusCodes(ImmutableList.of(500, 503))
+      .setRetryStatusCodes(ImmutableList.of(503))
       .setMaxIntervalMillis(60 * 1000)
       .build();
 

src/test/java/com/google/firebase/internal/ApiClientUtilsTest.java

@@ -66,7 +66,7 @@ public void testAuthorizedHttpClient() throws IOException {
     assertEquals(4, retryConfig.getMaxRetries());
     assertEquals(60 * 1000, retryConfig.getMaxIntervalMillis());
     assertFalse(retryConfig.isRetryOnIOExceptions());
-    assertEquals(retryConfig.getRetryStatusCodes(), ImmutableList.of(500, 503));
+    assertEquals(retryConfig.getRetryStatusCodes(), ImmutableList.of(503));
   }
 
   @Test