http: corrects undocumented side effects in http.DefaultTransport's RoundTrip method

Fixes #2140.

The http.DefaultTransport's RoundTrip method leaves the http.Request object
in an altered state after performing the round trip.  This patch removes
the header from the Request before returning to the client.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4857041

src/pkg/http/transport.go

@@ -511,6 +511,9 @@ func (pc *persistConn) readLoop() {
 			if err != nil || resp.ContentLength == 0 {
 				return resp, err
 			}
+			if rc.addedGzip {
+				forReq.Header.Del("Accept-Encoding")
+			}
 			if rc.addedGzip && resp.Header.Get("Content-Encoding") == "gzip" {
 				resp.Header.Del("Content-Encoding")
 				resp.Header.Del("Content-Length")

src/pkg/http/transport_test.go

@@ -387,6 +387,68 @@ func TestTransportNilURL(t *testing.T) {
 	}
 }
 
+var roundTripTests = []struct {
+	accept       string
+	expectAccept string
+	compressed   bool
+}{
+	// Requests with no accept-encoding header use transparent compression
+	{"", "gzip", false},
+	// Requests with other accept-encoding should pass through unmodified
+	{"foo", "foo", false},
+	// Requests with accept-encoding == gzip should be passed through
+	{"gzip", "gzip", true}}
+
+// Test that the modification made to the Request by the RoundTripper is cleaned up
+func TestRoundTripGzip(t *testing.T) {
+	const responseBody = "test response body"
+	ts := httptest.NewServer(HandlerFunc(func(rw ResponseWriter, req *Request) {
+		accept := req.Header.Get("Accept-Encoding")
+		if expect := req.FormValue("expect_accept"); accept != expect {
+			t.Errorf("Accept-Encoding = %q, want %q", accept, expect)
+		}
+		if accept == "gzip" {
+			rw.Header().Set("Content-Encoding", "gzip")
+			gz, _ := gzip.NewWriter(rw)
+			gz.Write([]byte(responseBody))
+			gz.Close()
+		} else {
+			rw.Header().Set("Content-Encoding", accept)
+			rw.Write([]byte(responseBody))
+		}
+	}))
+	defer ts.Close()
+
+	for i, test := range roundTripTests {
+		// Test basic request (no accept-encoding)
+		req, _ := NewRequest("GET", ts.URL+"?expect_accept="+test.expectAccept, nil)
+		req.Header.Set("Accept-Encoding", test.accept)
+		res, err := DefaultTransport.RoundTrip(req)
+		var body []byte
+		if test.compressed {
+			gzip, _ := gzip.NewReader(res.Body)
+			body, err = ioutil.ReadAll(gzip)
+			res.Body.Close()
+		} else {
+			body, err = ioutil.ReadAll(res.Body)
+		}
+		if err != nil {
+			t.Errorf("%d. Error: %q", i, err)
+		} else {
+			if g, e := string(body), responseBody; g != e {
+				t.Errorf("%d. body = %q; want %q", i, g, e)
+			}
+			if g, e := req.Header.Get("Accept-Encoding"), test.accept; g != e {
+				t.Errorf("%d. Accept-Encoding = %q; want %q", i, g, e)
+			}
+			if g, e := res.Header.Get("Content-Encoding"), test.accept; g != e {
+				t.Errorf("%d. Content-Encoding = %q; want %q", i, g, e)
+			}
+		}
+	}
+
+}
+
 func TestTransportGzip(t *testing.T) {
 	const testString = "The test string aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
 	const nRandBytes = 1024 * 1024