Incorrect expires_in value set by MockTokenServerTransport

[hiranya911]

https://github.com/google/google-api-java-client/blob/4fc8c099d9db5646770868cc1bc9a33c9225b3c7/google-api-client/src/main/java/com/google/api/client/googleapis/testing/auth/oauth2/MockTokenServerTransport.java#L119

expires_in is reported in seconds. Therefore it's more accurate to set 3600 here.