Remove the constraint that map keys mustn't be the empty string.

See bug 227.

gson/src/main/java/com/google/gson/JsonObject.java

@@ -52,7 +52,7 @@ public final class JsonObject extends JsonElement {
    * @param value the member object.
    */
   public void add(String property, JsonElement value) {
-    Preconditions.checkArgument(property != null && !"".equals(property.trim()));
+    Preconditions.checkArgument(property != null);
     if (value == null) {
       value = JsonNull.createJsonNull();
     }

gson/src/test/java/com/google/gson/JsonObjectTest.java

@@ -120,4 +120,20 @@ public class JsonObjectTest extends TestCase {
     String json = new Gson().toJson(jsonObj);
     assertEquals("{\"a\\\"b\":\"c\\\"d\"}", json);
   }
+
+  /**
+   * From issue 227.
+   */
+  public void testWritePropertyWithEmptyStringName() {
+    JsonObject jsonObj = new JsonObject();
+    jsonObj.add("", new JsonPrimitive(true));
+    assertEquals("{\"\":true}", new Gson().toJson(jsonObj));
+
+  }
+
+  public void testReadPropertyWithEmptyStringName() {
+    JsonObject jsonObj = new JsonParser().parse("{\"\":true}").getAsJsonObject();
+    assertEquals(true, jsonObj.get("").getAsBoolean());
+
+  }
 }

gson/src/test/java/com/google/gson/functional/MapTest.java

@@ -297,4 +297,19 @@ public class MapTest extends TestCase {
     String json = gson.toJson(map);
     assertEquals("{\"a\\\"b\":\"c\\\"d\"}", json);
   }
+  
+  /**
+   * From issue 227.
+   */
+  public void testWriteMapsWithEmptyStringKey() {
+    Map<String, Boolean> map = new HashMap<String, Boolean>();
+    map.put("", true);
+    assertEquals("{\"\":true}", gson.toJson(map));
+
+  }
+
+  public void testReadMapsWithEmptyStringKey() {
+    Map<String, Boolean> map = gson.fromJson("{\"\":true}", new TypeToken<Map<String, Boolean>>() {}.getType());
+    assertEquals(Boolean.TRUE, map.get(""));
+  }
 }