HangoutsChat - How can display button's text is only uppercase the first letter?

I tried to create an interactive message as Google Hangouts Chat spec, but my button's text always uppercase all letter. How can I display uppercase only the first letter?

https://developers.google.com/hangouts/chat/how-tos/cards-onclick

"buttons": [
  {
    "textButton": {
      "text": "Login",
      "onClick": {
        "action": {
          "actionMethodName": "loginMethod",
          "parameters": []
        }
      }
    }
  }
]

Expected: button text Login instead of LOGIN