compress/gzip: specify when Reader.Header is valid

The gzip package is asymmetrical in the way it handles headers.
In Writer, the Header is written on the first call to Write, Flush, or Close.
In Reader, the Header is read on calls to NewReader or Reset as opposed to
after the first Read. Thus, we document this difference.

Fixes #13211

Change-Id: I5f87beff036e5e2fd68a02a15fdb7137e9ca4c37
Reviewed-on: https://go-review.googlesource.com/16838Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/gunzip.go

@@ -69,7 +69,7 @@ type Header struct {
 // returned by Read as tentative until they receive the io.EOF
 // marking the end of the data.
 type Reader struct {
-	Header
+	Header       // valid after NewReader or Reader.Reset
 	r            flate.Reader
 	decompressor io.ReadCloser
 	digest       hash.Hash32
@@ -83,7 +83,10 @@ type Reader struct {
 // NewReader creates a new Reader reading the given reader.
 // If r does not also implement io.ByteReader,
 // the decompressor may read more data than necessary from r.
+//
 // It is the caller's responsibility to call Close on the Reader when done.
+//
+// The Reader.Header fields will be valid in the Reader returned.
 func NewReader(r io.Reader) (*Reader, error) {
 	z := new(Reader)
 	z.r = makeReader(r)

src/compress/gzip/gzip.go

@@ -25,7 +25,7 @@ const (
 // A Writer is an io.WriteCloser.
 // Writes to a Writer are compressed and written to w.
 type Writer struct {
-	Header
+	Header      // written at first call to Write, Flush, or Close
 	w           io.Writer
 	level       int
 	wroteHeader bool