/o/oauth2/token?grant_type=authorization_code&code=4%2F42sz3xxq4wGF9k9joYxRVop9mHi6.UpFaIUqZ_UQaaDn_6y0ZQNh869DgiwI&client_id=199079322117.apps.googleusercontent.com&client_secret=...&redirect_uri=...

Yeah I know its a very old question but I faced the same problem today.
and resolved the  issue as follow.
if you use form option it will automatically set content type
appticationx-ww-orm-urtencoded



  var request = require('request');
  request.post('https://accounts.google.com/o/oauth2/token',
      {
          form: {
              code: '4/M9YCOpJkweTzLKbkoDSj1JiCtFGp.otpz-eXAviMZgrKXntQAax12uvgYjwI',
              client_secret: 'BpQ-5ck5rpD0ixFmGYsEP998',
              client_id: '256393709562-l4idip7eh234kulhcfo6i7shcqmvnd7v.apps.googleusercontent.com',
              grant_type: 'authorization_code',
              redirect_uri: 'https://developers.google.com/oauthplayground'
          }
     },function(err,res,body){
         console.log(body);
         if(err) console.log(err);
         //console.log(res);
     });