Put sync.Mutex on top of variables it protects.

[dmitshur]

This change makes the code more consistent with the rateMu below.

This is a followup to #390 (comment).

I've also considered an alternative solution that kept the comments on top

// clientMu protects the client during calls that modify the CheckRedirect func.
clientMu sync.Mutex
// HTTP client used to communicate with the API.
client *http.Client

or

clientMu sync.Mutex // clientMu protects the client during calls that modify the CheckRedirect func.
// HTTP client used to communicate with the API.
client *http.Client

but decided to go with the inline version because I figured these are internal implementation details and unexported variables, so it's fine.

If there's any preference towards keeping the comments on top, I'm happy to change it back.

[gmlewis]

LGTM 👍
Awaiting second LGTM before merging.

[gmlewis]

Incorporated in 136ef5c