title: Storing enum with room database gives error even with TypeConverters
body: <p>I've followed the room documentation on how to add TypeConverters for custom types, but I still get an error from my entity class. I'd like to simply convert the <code>Category</code> enum to a <code>String</code> so the room database understands how to store it. Below, every <code>Exercise</code> has a category, and this is where the error occurs. Here are my classes:</p>
  
  <p>Converter</p>
  
  <pre><code>public class Converter {
      @TypeConverter
      public static String fromCategoryToString(Category category) {
          if (category == null)
              return null;
          return category.toString();
      }
  }
  </code></pre>
  
  <p>Category </p>
  
  <pre><code>public enum Category {
      EXERCISE("Exercise"),
      REST("Rest"),
      COUNTDOWN("Countdown");
  
      private final String text;
  
      Category(final String text) {
          this.text = text;
      }
  
      @Override
      public String toString() {
          return text;
      }
  }
  </code></pre>
  
  <p>Exercise. Getters and setters removed for brevity.</p>
  
  <pre><code>@Entity(tableName = "exercise",
          foreignKeys = @ForeignKey(entity = Routine.class,
                  parentColumns = "rid",
                  childColumns = "routineId",
                  onDelete = ForeignKey.CASCADE),
          indices = {@Index(value = "name", unique = true), @Index("routineId")})
  public class Exercise {
      @PrimaryKey(autoGenerate = true)
      @ColumnInfo(name = "eid")
      private int exerciseID;
  
      @ColumnInfo(name = "category")
      private Category category; // error occurs here
  
      @ColumnInfo(name = "name")
      @NonNull
      private String name;
  
      @ColumnInfo(name = "routineId")
      private int routineId;
  
      public Exercise(Category category, @NonNull String name) {
          this.category = category;
          this.name = name;
      }
  
  }
  </code></pre>
  
  <p>Database</p>
  
  <pre><code>@Database(entities = { Routine.class, Exercise.class}, version = 1)
  @TypeConverters({ Converter.class })
  public abstract class AppDatabase extends RoomDatabase {
      private static AppDatabase instance;
      private static final String dbName = "routines.db";
  
      public abstract RoutineDao routineDao();
      public abstract ExerciseDao exerciseDao();
  
      public static AppDatabase getInstance(final Context context) {
          if (instance == null) {
              instance = Room.databaseBuilder(context.getApplicationContext(), AppDatabase.class, dbName).build();
          }
          return instance;
      }
  
  
  }
  </code></pre>