Can't set headers after they are sent after 2.2.17 upgrade

[cspnanda]

Issue Description

• Deployed Parse Server 2.2.17 on Heroku.
• Using iOS SDK. I set in a Parse Object and save it as

[pfObject saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) { }]

In the heroku logs I see

Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:346:11)
at /app/node_modules/express/lib/application.js:153:9
at /app/node_modules/express/lib/router/index.js:137:5
at /app/node_modules/express/lib/router/index.js:250:10
at next (/app/node_modules/express/lib/router/index.js:160:14)
at next (/app/node_modules/express/lib/router/index.js:176:38)
at Immediate. (/app/node_modules/express/lib/application.js:190:9)
at Immediate. (/app/node_modules/parse-server/node_modules/express/lib/router/index.js:618:15)
at Immediate.immediate._onImmediate (timers.js:572:18)
at tryOnImmediate (timers.js:534:15)

This was not happening before.

[maziyarpanahi]

#2362

[woodardj]

A touch hacky, but we included something like this immediately after mounting the parse-server middleware:

app.use(function (req, res, next) {
    var path = req.path.split('/');
    if (path[1] === 'parse') {
        return;
    }
    return next();
});

This let us move forward with fixing the rest of our bugs while halting requests to the parse API that we weren't expecting to flow through (as was the case on Parse.com), and we can fix the rest of our app to expect this more standard Express behavior later.