If the requested url does not have a protocol (http/https) specified, the error given is a bit cryptic. While it is a bit of an edge case, it has actually happened to me on numerous occasions.
Currently, the following is thrown:

```
error checking auth TypeError: Cannot read property 'slice' of null
    at dispatchHttpRequest (.../node_modules/axios/lib/adapters/http.js:84:37)
    at httpAdapter (.../node_modules/axios/lib/adapters/http.js:19:10)
    at dispatchRequest (.../node_modules/axios/lib/core/dispatchRequest.js:52:10)
    at process._tickCallback (internal/process/next_tick.js:103:7)
```

The reason I'm opening a issue rather than a pull request is that I see two different ways of handling it and would like @mzabriskie to chime in.
1. Check explicitly for `parsed.protocol` in the httpAdapter and throw an error if it doesn't exist.
2. Default to using `http://` and continue on as usual

Love axios, long time user, let me know what you think and I can make a PR.

lib/adapters/http.js
var settle = require('./../core/settle');
var buildURL = require('./../helpers/buildURL');
var http = require('http');
var https = require('https');
var httpFollow = require('follow-redirects').http;
var httpsFollow = require('follow-redirects').https;
var url = require('url');
var zlib = require('zlib');
var pkg = require('./../../package.json');
var Buffer = require('buffer').Buffer;
var createError = require('../core/createError');
var enhanceError = require('../core/enhanceError');
module.exports = function httpAdapter(config) {
  return new Promise(function dispatchHttpRequest(resolve, reject) {
    var data = config.data;
    var headers = config.headers;
    var timer;
    var aborted = false;
    
    
    
    if (!headers['User-Agent'] && !headers['user-agent']) {
      headers['User-Agent'] = 'axios/' + pkg.version;
    }
    if (data && !utils.isStream(data)) {
      if (utils.isArrayBuffer(data)) {
        data = new Buffer(new Uint8Array(data));
      } else if (utils.isString(data)) {
        data = new Buffer(data, 'utf-8');
      } else {