Support stores with configurations defined as objects without prototype

[kosmasgiannis]

If a memory store is added where the configuration is an object without prototype, when you try to get something the following error occurs:

TypeError: target.hasOwnProperty is not a function

Here is an example to reproduce:

var nconf= require('nconf');

var np = Object.assign(Object.create(null), {key1:'value1'});

nconf.add('np', {type: 'literal', store: np});

console.log(nconf.get('key1'));

[ilkkao]

Most importantly ini bump to 2.0.0 includes this change https://github.com/npm/ini/commit/032fbaf5f0b98fce70c8cc380e0d05177a9c9073

Which in my project with

  nconf.file('defaults', {
    file: path.join(rootPath, 'server', 'mas.conf.default'),
    format: nconf.formats.ini
  });

leads to the same

/Users/ilkkao/git/mas/server/node_modules/nconf/lib/nconf/stores/memory.js:47
      if (target && target.hasOwnProperty(key)) {
                           ^
  TypeError: target.hasOwnProperty is not a function
     at exports.File.Memory.get (/Users/ilkkao/git/mas/server/node_modules/nconf/lib/nconf/stores/memory.js:47:26)
     at /Users/ilkkao/git/mas/server/node_modules/nconf/lib/nconf/provider.js:629:32
     at Array.forEach (<anonymous>)