Extend DialOptions to allow Host header override

Commit from coder#336

.gitignore

@@ -1 +1,2 @@
 websocket.test
+.idea

dial.go

@@ -1,3 +1,4 @@
+//go:build !js
 // +build !js
 
 package websocket
@@ -30,6 +31,10 @@ type DialOptions struct {
 	// HTTPHeader specifies the HTTP headers included in the handshake request.
 	HTTPHeader http.Header
 
+	// Host optionally overrides the Host HTTP header to send. If empty, the value
+	// of URL.Host will be used.
+	Host string
+
 	// Subprotocols lists the WebSocket subprotocols to negotiate with the server.
 	Subprotocols []string
 
@@ -158,6 +163,9 @@ func handshakeRequest(ctx context.Context, urls string, opts *DialOptions, copts
 	}
 
 	req, _ := http.NewRequestWithContext(ctx, "GET", u.String(), nil)
+	if len(opts.Host) > 0 {
+		req.Host = opts.Host
+	}
 	req.Header = opts.HTTPHeader.Clone()
 	req.Header.Set("Connection", "Upgrade")
 	req.Header.Set("Upgrade", "websocket")

dial_test.go

@@ -1,3 +1,4 @@
+//go:build !js
 // +build !js
 
 package websocket
@@ -108,6 +109,56 @@ func TestBadDials(t *testing.T) {
 	})
 }
 
+func Test_verifyHostOverride(t *testing.T) {
+	testCases := []struct {
+		name string
+		host string
+		exp  string
+	}{
+		{
+			name: "noOverride",
+			host: "",
+			exp:  "example.com",
+		},
+		{
+			name: "hostOverride",
+			host: "example.net",
+			exp:  "example.net",
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
+			defer cancel()
+
+			rt := func(r *http.Request) (*http.Response, error) {
+				assert.Equal(t, "Host", tc.exp, r.Host)
+
+				h := http.Header{}
+				h.Set("Connection", "Upgrade")
+				h.Set("Upgrade", "websocket")
+				h.Set("Sec-WebSocket-Accept", secWebSocketAccept(r.Header.Get("Sec-WebSocket-Key")))
+
+				return &http.Response{
+					StatusCode: http.StatusSwitchingProtocols,
+					Header:     h,
+					Body:       ioutil.NopCloser(strings.NewReader("hi")),
+				}, nil
+			}
+
+			_, _, _ = Dial(ctx, "ws://example.com", &DialOptions{
+				HTTPClient: mockHTTPClient(rt),
+				Host:       tc.host,
+			})
+		})
+	}
+
+}
+
 func Test_verifyServerHandshake(t *testing.T) {
 	t.Parallel()