proxy: preserve path trailing slash if it was there

fix issue #1177
Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/proxy/proxy_test.go

@@ -791,6 +791,11 @@ func TestProxyDirectorURL(t *testing.T) {
 			expectURL:  `https://localhost:2021/t?foo%3dbar&t%3dw`,
 			without:    "/test",
 		},
+		{
+			requestURL: `http://localhost:2020/test/`,
+			targetURL:  `https://localhost:2021/t/`,
+			expectURL:  `https://localhost:2021/t/test/`,
+		},
 	} {
 		targetURL, err := url.Parse(c.targetURL)
 		if err != nil {

caddyhttp/proxy/reverseproxy.go

@@ -96,7 +96,13 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 			}
 		}
 
+		hadTrailingSlash := strings.HasSuffix(req.URL.Path, "/")
 		req.URL.Path = path.Join(target.Path, req.URL.Path)
+		// path.Join will strip off the last /, so put it back if it was there.
+		if hadTrailingSlash && !strings.HasSuffix(req.URL.Path, "/") {
+			req.URL.Path = req.URL.Path + "/"
+		}
+
 		// Trims the path of the socket from the URL path.
 		// This is done because req.URL passed to your proxied service
 		// will have the full path of the socket file prefixed to it.