Fix broken JSON support in cli/command/formatter

How to test:

    $ docker ps --format '{{json .}}'
    $ docker network ls --format '{{json .}}'
    $ docker volume ls --format '{{json .}}'

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

pkg/testutil/assert/assert.go

@@ -4,6 +4,7 @@ package assert
 import (
 	"fmt"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strings"
 )
@@ -44,6 +45,14 @@ func NilError(t TestingT, err error) {
 	}
 }
 
+// DeepEqual compare the actual value to the expected value and fails the test if
+// they are not "deeply equal".
+func DeepEqual(t TestingT, actual, expected interface{}) {
+	if !reflect.DeepEqual(actual, expected) {
+		fatal(t, "Expected '%v' (%T) got '%v' (%T)", expected, expected, actual, actual)
+	}
+}
+
 // Error asserts that error is not nil, and contains the expected text,
 // otherwise it fails the test.
 func Error(t TestingT, err error, contains string) {