Add support for decoding JSON arrays of arrays

Add support for decoding JSON arrays of arrays by handling, during
slice decoding, when the next token is an array opening. This produces
nested []interface{} slices.

Closes getsops#640.

stores/json/store.go

@@ -79,6 +79,12 @@ func (store Store) sliceFromJSONDecoder(dec *json.Decoder) ([]interface{}, error
 				return slice, err
 			}
 			slice = append(slice, item)
+		} else if ok && delim.String() == "[" {
+			item, err := store.sliceFromJSONDecoder(dec)
+			if err != nil {
+				return slice, err
+			}
+			slice = append(slice, item)
 		} else {
 			slice = append(slice, t)
 		}

stores/json/store_test.go

@@ -198,6 +198,31 @@ func TestDecodeJSONArrayOfObjects(t *testing.T) {
 	assert.Equal(t, expected, branch)
 }
 
+func TestDecodeJSONArrayOfArrays(t *testing.T) {
+	in := `{"foo": [[["foo", {"bar": "foo"}]]]}`
+	expected := sops.TreeBranch{
+		sops.TreeItem{
+			Key: "foo",
+			Value: []interface{}{
+				[]interface{}{
+					[]interface{}{
+						"foo",
+						sops.TreeBranch{
+							sops.TreeItem{
+								Key:   "bar",
+								Value: "foo",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+	branch, err := Store{}.treeBranchFromJSON([]byte(in))
+	assert.Nil(t, err)
+	assert.Equal(t, expected, branch)
+}
+
 func TestEncodeSimpleJSON(t *testing.T) {
 	branch := sops.TreeBranch{
 		sops.TreeItem{