Fix json serialization

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/auth/StoredVerificationCode.java

@@ -8,10 +8,12 @@ import java.util.concurrent.TimeUnit;
 public class StoredVerificationCode {
 
   @JsonProperty
-  private final String code;
+  private String code;
 
   @JsonProperty
-  private final long   timestamp;
+  private long   timestamp;
+
+  public StoredVerificationCode() {}
 
   public StoredVerificationCode(String code, long timestamp) {
     this.code      = code;

src/main/java/org/whispersystems/textsecuregcm/storage/PendingAccountsManager.java

@@ -86,7 +86,7 @@ public class PendingAccountsManager {
       if (json == null) return Optional.absent();
       else              return Optional.of(mapper.readValue(json, StoredVerificationCode.class));
     } catch (IOException e) {
-      logger.warn("PendingAccountsManager", "Error deserializing value...");
+      logger.warn("Error deserializing value...", e);
       return Optional.absent();
     }
   }