net/http/httptest: deprecate ResponseRecorder.HeaderMap

Users of this field are better off using Result instead.

Fixes #25763.

Change-Id: I4391afa6ed3873107628630adc1d409d77fb3f20
Reviewed-on: https://go-review.googlesource.com/117675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httptest/recorder.go

@@ -27,9 +27,11 @@ type ResponseRecorder struct {
 	Code int
 
 	// HeaderMap contains the headers explicitly set by the Handler.
+	// It is an internal detail.
 	//
-	// To get the implicit headers set by the server (such as
-	// automatic Content-Type), use the Result method.
+	// Deprecated: HeaderMap exists for historical compatibility
+	// and should not be used. To access the headers returned by a handler,
+	// use the Response.Header map as returned by the Result method.
 	HeaderMap http.Header
 
 	// Body is the buffer to which the Handler's Write calls are sent.