allow simple strings inside ngettext

[AlexMost]

Need to think about this kind of plurals, maybe it's better to omit template literals where we don't need them.

ERROR in ./src/ngettextDemo.js
Module build failed: SyntaxError: Cannot read property 'forEach' of undefined

  20 |                 </div>
  21 |                 <button onClick={this.countInc}>{ t`Click me` }</button>
> 22 | 		<div>{ngettext(msgid`test`, 'tests', n)}</div>
     | 		      ^
  23 |             </div>
  24 |         )
  25 |     }