http: initialize request Header for the transport

Fixes #1558

R=rsc, r, bradfitzwork
CC=golang-dev
https://golang.org/cl/4260042

src/pkg/http/client.go

@@ -36,6 +36,9 @@ type ClientTransport interface {
 	// be reserved for failure to obtain a response.  Similarly, Do should
 	// not attempt to handle higher-level protocol details such as redirects,
 	// authentication, or cookies.
+	//
+	// Transports may modify the request. The request Headers field is
+	// guaranteed to be initalized.
 	Do(req *Request) (resp *Response, err os.Error)
 }
 
@@ -109,6 +112,14 @@ func send(req *Request, t ClientTransport) (resp *Response, err os.Error) {
 			return
 		}
 	}
+
+	// Most the callers of send (Get, Post, et al) don't need
+	// Headers, leaving it uninitialized.  We guarantee to the
+	// ClientTransport that this has been initialized, though.
+	if req.Header == nil {
+		req.Header = Header(make(map[string][]string))
+	}
+
 	info := req.URL.RawUserinfo
 	if len(info) > 0 {
 		enc := base64.URLEncoding

src/pkg/http/client_test.go

@@ -55,9 +55,12 @@ func TestGetRequestFormat(t *testing.T) {
 	url := "http://dummy.faketld/"
 	client.Get(url) // Note: doesn't hit network
 	if tr.req.Method != "GET" {
-		t.Fatalf("expected method %q; got %q", "GET", tr.req.Method)
+		t.Errorf("expected method %q; got %q", "GET", tr.req.Method)
 	}
 	if tr.req.URL.String() != url {
-		t.Fatalf("expected URL %q; got %q", url, tr.req.URL.String())
+		t.Errorf("expected URL %q; got %q", url, tr.req.URL.String())
+	}
+	if tr.req.Header == nil {
+		t.Errorf("expected non-nil request Header")
 	}
 }