Ignore directories in disk resolver (#241)

The disk resolver was the only one that returned directories as box
files. This modification allows http redirections of index.html to
work when working with the default disk resolver.

A test case has been added to verify http's behavior when working with
the disk resolver.

Fix #162
Fix #211
Fix #235

Author: Lesterpig

v2/_fixtures/http_test/css/main.css

@@ -0,0 +1 @@
+Css

v2/_fixtures/http_test/footer.html

@@ -0,0 +1 @@
+Footer

v2/_fixtures/http_test/index.html

@@ -0,0 +1 @@
+Index

v2/_fixtures/http_test/sub/index.html

@@ -0,0 +1 @@
+Sub

v2/file/resolver/disk.go

@@ -37,7 +37,7 @@ func (d *Disk) Resolve(box string, name string) (file.File, error) {
 		return nil, err
 	}
 	if fi.IsDir() {
-		return file.NewDir(OsPath(name))
+		return nil, os.ErrNotExist
 	}
 	if bb, err := ioutil.ReadFile(path); err == nil {
 		return file.NewFile(OsPath(name), bb)

v2/http_box_test.go

@@ -25,8 +25,8 @@ var httpBox = func() packd.Box {
 	}
 
 	hg, err := resolver.NewHexGzip(map[string]string{
-		"index.html":	ind,
-		"hello.txt":	hello,
+		"index.html": ind,
+		"hello.txt":  hello,
 	})
 	if err != nil {
 		panic(err)
@@ -129,3 +129,41 @@ func Test_HTTPBox_CaseInsensitive(t *testing.T) {
 		})
 	}
 }
+
+func Test_HTTPBox_Disk(t *testing.T) {
+	r := require.New(t)
+
+	box := New("http disk box", "./_fixtures/http_test")
+	mux := http.NewServeMux()
+	mux.Handle("/", http.FileServer(box))
+
+	type testcase struct {
+		URL, Content, Location string
+		Code                   int
+	}
+
+	testcases := []testcase{
+		{"/", "Index", "", 200},
+		{"/sub", "Sub", "", 200},
+		{"/index.html", "", "./", 301},
+		{"/sub/index.html", "", "./", 301},
+		{"/sub/", "", "../sub", 301},
+		{"/footer.html", "Footer", "", 200},
+		{"/css/main.css", "Css", "", 200},
+		{"/css", "404 page not found", "", 404},
+		{"/css/", "404 page not found", "", 404},
+	}
+
+	for _, tc := range testcases {
+		t.Run("path"+tc.URL, func(t *testing.T) {
+			req, err := http.NewRequest("GET", tc.URL, nil)
+			r.NoError(err)
+			res := httptest.NewRecorder()
+			mux.ServeHTTP(res, req)
+
+			r.Equal(tc.Code, res.Code)
+			r.Equal(tc.Location, res.Header().Get("location"))
+			r.Equal(tc.Content, strings.TrimSpace(res.Body.String()))
+		})
+	}
+}