context: produce a nicer panic message for a nil WithValue key

Change-Id: I2e8ae403622ba7131cadaba506100d79613183f1
Reviewed-on: https://go-review.googlesource.com/22601
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/context/context.go

@@ -428,6 +428,9 @@ func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
 //
 // The provided key must be comparable.
 func WithValue(parent Context, key, val interface{}) Context {
+	if key == nil {
+		panic("nil key")
+	}
 	if !reflect.TypeOf(key).Comparable() {
 		panic("key is not comparable")
 	}

src/context/context_test.go

@@ -583,6 +583,10 @@ func TestWithValueChecksKey(t *testing.T) {
 	if panicVal == nil {
 		t.Error("expected panic")
 	}
+	panicVal = recoveredValue(func() { WithValue(Background(), nil, "bar") })
+	if got, want := fmt.Sprint(panicVal), "nil key"; got != want {
+		t.Errorf("panic = %q; want %q", got, want)
+	}
 }
 
 func recoveredValue(fn func()) (v interface{}) {