impersonator: test UID impersonation and header canonicalization

Signed-off-by: Monis Khan <mok@vmware.com>

internal/httputil/roundtripper/roundtripper.go

@@ -0,0 +1,14 @@
+// Copyright 2021 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package roundtripper
+
+import "net/http"
+
+var _ http.RoundTripper = Func(nil)
+
+type Func func(*http.Request) (*http.Response, error)
+
+func (f Func) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}