Node version (or tell us if you're using electron or some other framework):6.11

ShellJS version (the most recent version/Github branch you see the bug on):0.7.8

Operating system:windows10

Description of the bug:Cannot set property to of [object String] which has only a getter

Example ShellJS command to reproduce the error: shelljs.rm

gulp.task('clean', () => {
  if (fs.existsSync(resolveCwd('lib'))) {
    shelljs.rm('-rf', resolveCwd('lib'));
  }
});

the error details is：
pipeMethods: [ 'cat','exec','grep', 'head', 'sed','sort','tail', 'to', 'toEnd', 'uniq' ]
cmd cat
cmd exec
cmd grep
cmd head
cmd sed
cmd sort
cmd tail
cmd to
error TypeError: Cannot set property to of [object String] which has only a getter
cmd toEnd
cmd uniq

that is to say, the 'to' method cannot set property!