allow expires_in string type

[csbok]

naver provider's response is below.

{
  access_token: '...',
  refresh_token: '...',
  token_type: 'bearer',
  expires_in: '3600'
}

expires_in is a string type. so I modified a if condition that allow string type.

[panva]

expires_in in JSON responses is defined as a number, please reach out to your provider to get a fix on their end instead.