LDP crashes on requests with no Accept in the headers

[nicola]

The issue is in line https://github.com/bergos/ldp/blob/master/ldp.js#L121

If the req.headers.accept is not set, then, it will fail to parse it/find out what mime type it is.
So, we should add a options.defaultAccept or options.defaultMimeType to be text/ttl

TypeError: Cannot call method 'split' of undefined
    at parseRanges (/Users/mozilla/Proj/gits/node-solid/node_modules/ldp/node_modules/mimeparse/lib/mimeparse.js:161:48)
    at Object.exports.bestMatch (/Users/mozilla/Proj/gits/node-solid/node_modules/ldp/node_modules/mimeparse/lib/mimeparse.js:148:24)
    at Object.self.serializers.find (/Users/mozilla/Proj/gits/node-solid/node_modules/ldp/ldp.js:35:30)
    at self.get (/Users/mozilla/Proj/gits/node-solid/node_modules/ldp/ldp.js:97:37)
    at self.middleware (/Users/mozilla/Proj/gits/node-solid/node_modules/ldp/ldp.js:81:12)
    at Layer.handle [as handle_request] (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:312:13)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:330:12)
    at next (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:271:10)
    at cors (/Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:178:7)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:228:17
    at originCallback (/Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:217:15)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:222:13
    at optionsCallback (/Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:203:9)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/cors/lib/index.js:208:7
    at Layer.handle [as handle_request] (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:312:13)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:330:12)
    at next (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:271:10)
    at expressInit (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/middleware/init.js:33:5)
    at Layer.handle [as handle_request] (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:312:13)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:330:12)
    at next (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:271:10)
    at query (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/middleware/query.js:49:5)
    at Layer.handle [as handle_request] (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:312:13)
    at /Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:330:12)
    at next (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:271:10)
    at Function.handle (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/router/index.js:176:3)
    at EventEmitter.handle (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/application.js:173:10)
    at Server.app (/Users/mozilla/Proj/gits/node-solid/node_modules/express/lib/express.js:38:9)
    at Server.emit (events.js:98:17)
    at HTTPParser.parser.onIncoming (http.js:2109:12)
    at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:122:23)
    at Socket.socket.ondata (http.js:1967:22)
    at TCP.onread (net.js:528:27)