Used `add` instead of `+=` for `Resources::filter`.

The current logic of `Resources::filter` is using `+=` to add
the filtered resource object and the `+=` resource object will
invoke `validate` which is not necessary as all of the resource
objects are valid.

The fix is using `add` instead of `+=` for `Resources::filter`,
this can make sure there is no validation and can improve
performance of `Resources::filter` for resources object.

Review: https://reviews.apache.org/r/50557/
diff --git a/src/common/resources.cpp b/src/common/resources.cpp
index 1fe7909..69166de 100644
--- a/src/common/resources.cpp
+++ b/src/common/resources.cpp
@@ -849,7 +849,7 @@
   Resources result;
   foreach (const Resource& resource, resources) {
     if (predicate(resource)) {
-      result += resource;
+      result.add(resource);
     }
   }
   return result;
diff --git a/src/v1/resources.cpp b/src/v1/resources.cpp
index c62bc35..90897e3 100644
--- a/src/v1/resources.cpp
+++ b/src/v1/resources.cpp
@@ -852,7 +852,7 @@
   Resources result;
   foreach (const Resource& resource, resources) {
     if (predicate(resource)) {
-      result += resource;
+      result.add(resource);
     }
   }
   return result;