all: use stdlib context instead of x/net/context

This PR replaces use of `x/net/context` with the standard `context`

It has been nearly 6 months since
https://github.com/golang/oauth2/issues/246#issuecomment-387601277 so
I made this PR so it will be ready to merge when needed (and if
possible).

Fixes #246

Change-Id: Id2c316fcb27de0fb9163ceb4e8669b04cb39a987
GitHub-Last-Rev: 5b36321dcc
GitHub-Pull-Request: golang/oauth2#339
Reviewed-on: https://go-review.googlesource.com/c/145202
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2_test.go

@@ -5,6 +5,7 @@
 package oauth2
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -13,8 +14,6 @@ import (
 	"net/url"
 	"testing"
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 type mockTransport struct {