internal: Use provided context in subsequent request

Currently the HTTP request does not set the given context.
This change sets the context (if not nil) on the request.

Change-Id: I4bb21636d05050a68ba70ce92f9bf9ba608fbfad
Reviewed-on: https://go-review.googlesource.com/45370
Run-TryBot: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

internal/token.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"golang.org/x/net/context"
+	"golang.org/x/net/context/ctxhttp"
 )
 
 // Token represents the crendentials used to authorize
@@ -189,7 +190,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 	if !bustedAuth {
 		req.SetBasicAuth(clientID, clientSecret)
 	}
-	r, err := hc.Do(req)
+	r, err := ctxhttp.Do(ctx, hc, req)
 	if err != nil {
 		return nil, err
 	}