Automatic permissions request Android Instrumentation Tests

I am doing my first Android Instrumentation tests and some of them needs permissions as Internet or Write External Storage. Following new Marshmallow permission way,I defined these permissions in my AndroidManifest.xml and defined a rule in my test like:

@Rule
public final PermissionsRule permissionsRule = new PermissionsRule(
        new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE,Manifest.permission.READ_EXTERNAL_STORAGE});

PermissionsRule is:

public class PermissionsRule implements TestRule {

    private final String[] permissions;

    public PermissionsRule(String[] permissions) {
        this.permissions = permissions;
    }

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {

                allowPermissions();

                base.evaluate();

                revokePermissions();
            }
        };
    }



    private void allowPermissions() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            for (String permission : permissions) {
                System.out.println("allowPermissions "+permission);
                getInstrumentation().getUiAutomation().executeShellCommand(
                        "pm grant " + InstrumentationRegistry.getTargetContext().getPackageName()
                                + " " + permission);
            }
        }
    }

    private void revokePermissions() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            for (String permission : permissions) {
                getInstrumentation().getUiAutomation().executeShellCommand(
                        "pm revoke " + InstrumentationRegistry.getTargetContext().getPackageName()
                                + " " + permission);
            }
        }
    }
}

Internet permission runs fine but write or read no. why?