Merge pull request #39 from cboudereau/fix/issue37

fix duration microseconds convertion

src/gcp/oauth2/token.rs

@@ -31,7 +31,7 @@ where
  D: Deserializer<'de>,
 {
  let expires_in: i64 = Deserialize::deserialize(deserializer)?;
- Ok(Utc::now() + chrono::Duration::microseconds(expires_in * 1000_1000))
+ Ok(Utc::now() + chrono::Duration::microseconds(expires_in * ONE_SECOND_TO_MICROSECONDS))
 }
 
 impl Display for Token {