API: fix birthday format in responses

app/presenters/profile_presenter.rb

@@ -65,7 +65,7 @@ class ProfilePresenter < BasePresenter
 
   def added_details_api_json
     {
-      birthday: formatted_birthday,
+      birthday: birthday.try(:iso8601),
       gender:   gender,
       location: location_message.plain_text_for_json,
       bio:      bio_message.plain_text_for_json

spec/integration/api/users_controller_spec.rb

@@ -191,7 +191,7 @@ describe Api::V1::UsersController do
     context "Full with all valid fields" do
       it "succeeds" do
         new_bio = "new bio"
-        new_birthday = Time.current + 8_640_000
+        new_birthday = Date.current + 100
         new_gender = "ask1"
         new_location = "new location"
         new_first_name = "new first"
@@ -220,7 +220,7 @@ describe Api::V1::UsersController do
         user = response_body(response)
         confirm_self_data_format(user)
         expect(user["bio"]).to eq(new_bio)
-        expect(user["birthday"]).to eq(birthday_format(new_birthday))
+        expect(user["birthday"]).to eq(new_birthday.iso8601)
         expect(user["location"]).to eq(new_location)
         expect(user["gender"]).to eq(new_gender)
         expect(user["first_name"]).to eq(new_first_name)