Add URI encoding to mongo auth parameters

The mongodb driver requires auth values be URI encoded:
044063097d

This uses node's built-in url module to encode the auth portion, by
parsing and re-formatting it, which causes special characters to get URI
encoded properly:
https://nodejs.org/api/url.html#url_escaped_characters

This is all a bit silly since mongodb just takes our passed uri, and
runs it through the same url parser again, but not before explicitly
erroring on '@' characters in the uri.

This is similiar to #148 (reverted by #297), but with much less code,
and hopefully less breakage. Also, note that `uri_decode_auth` is no
longer needed. That was removed in the above referenced
node-mongodb-native commit.

I've tested this on usernames and passwords with @, !, +, and a space.

Presumably this would also work with usernames and passwords that are
already URI encoded (since parseUrl will simply unescape it, and
formatUrl will escape it again).

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -1,6 +1,7 @@
 
 import MongoCollection from './MongoCollection';
 import MongoSchemaCollection from './MongoSchemaCollection';
+import {parse as parseUrl, format as formatUrl} from 'url';
 
 let mongodb = require('mongodb');
 let MongoClient = mongodb.MongoClient;
@@ -25,7 +26,11 @@ export class MongoStorageAdapter {
       return this.connectionPromise;
     }
 
-    this.connectionPromise = MongoClient.connect(this._uri, this._options).then(database => {
+    // parsing and re-formatting causes the auth value (if there) to get URI
+    // encoded
+    const encodedUri = formatUrl(parseUrl(this._uri));
+
+    this.connectionPromise = MongoClient.connect(encodedUri, this._options).then(database => {
       this.database = database;
     });
     return this.connectionPromise;