http: change ResponseWriter.SetHeader(k,v) to Header() accessor

Caller code needs to change:

rw.SetHeader("Content-Type", "text/plain")
to:
rw.Header().Set("Content-Type", "text/plain")

This now permits returning multiple headers
with the same name using Add:

rw.Header().Add("Set-Cookie", "..")
rw.Header().Add("Set-Cookie", "..")

This patch also fixes serialization of headers, removing newline characters.

Fixes #488
Fixes #914

R=rsc
CC=gburd, golang-dev
https://golang.org/cl/4239076

src/pkg/http/responsewrite_test.go

@@ -65,6 +65,29 @@ var respWriteTests = []respWriteTest{
 			"Transfer-Encoding: chunked\r\n\r\n" +
 			"6\r\nabcdef\r\n0\r\n\r\n",
 	},
+
+	// Header value with a newline character (Issue 914).
+	// Also tests removal of leading and trailing whitespace.
+	{
+		Response{
+			StatusCode:    204,
+			ProtoMajor:    1,
+			ProtoMinor:    1,
+			RequestMethod: "GET",
+			Header: Header{
+				"Foo": []string{" Bar\nBaz "},
+			},
+			Body:             nil,
+			ContentLength:    0,
+			TransferEncoding: []string{"chunked"},
+			Close:            true,
+		},
+
+		"HTTP/1.1 204 No Content\r\n" +
+			"Connection: close\r\n" +
+			"Foo: Bar Baz\r\n" +
+			"\r\n",
+	},
 }
 
 func TestResponseWrite(t *testing.T) {
@@ -78,7 +101,7 @@ func TestResponseWrite(t *testing.T) {
 		}
 		sraw := braw.String()
 		if sraw != tt.Raw {
-			t.Errorf("Test %d, expecting:\n%s\nGot:\n%s\n", i, tt.Raw, sraw)
+			t.Errorf("Test %d, expecting:\n%q\nGot:\n%q\n", i, tt.Raw, sraw)
 			continue
 		}
 	}