Make API key less likely to be logged (#83)

Before this change:

    const at = new Airtable({ apiKey: 'keyXyz' })
    console.log(at)
    // => Class { _apiKey: 'keyXyz', ... }

After this change:

    const at = new Airtable({ apiKey: 'keyXyz' })
    console.log(at)
    // => Class { ... }

This is accomplished with `Object.defineProperties`. By default,
properties defined this way aren't enumerable, making it more difficult
to accidentally log them.

Addresses <Airtable/airtable.js#82>.

lib/airtable.js

@@ -12,15 +12,32 @@ var Airtable = Class.extend({
     init: function(opts) {
         opts = opts || {};
 
-        var default_config = Airtable.default_config();
-
-        this._apiKey = opts.apiKey || Airtable.apiKey || default_config.apiKey;
-        this._endpointUrl = opts.endpointUrl || Airtable.endpointUrl || default_config.endpointUrl;
-        this._apiVersion = opts.apiVersion || Airtable.apiVersion || default_config.apiVersion;
-        this._apiVersionMajor = this._apiVersion.split('.')[0];
-        this._allowUnauthorizedSsl = opts.allowUnauthorizedSsl || Airtable.allowUnauthorizedSsl || default_config.allowUnauthorizedSsl;
-        this._noRetryIfRateLimited = opts.noRetryIfRateLimited || Airtable.noRetryIfRateLimited || default_config.noRetryIfRateLimited;
-        this.requestTimeout = opts.requestTimeout || default_config.requestTimeout;
+        var defaultConfig = Airtable.default_config();
+
+        var apiVersion = opts.apiVersion || Airtable.apiVersion || defaultConfig.apiVersion;
+
+        Object.defineProperties(this, {
+            _apiKey: {
+                value: opts.apiKey || Airtable.apiKey || defaultConfig.apiKey,
+            },
+            _endpointUrl: {
+                value: opts.endpointUrl || Airtable.endpointUrl || defaultConfig.endpointUrl,
+            },
+            _apiVersion: {
+                value: apiVersion,
+            },
+            _apiVersionMajor: {
+                value: apiVersion.split('.')[0],
+            },
+            _allowUnauthorizedSsl: {
+                value: opts.allowUnauthorizedSsl || Airtable.allowUnauthorizedSsl || defaultConfig.allowUnauthorizedSsl,
+            },
+            _noRetryIfRateLimited: {
+                value: opts.noRetryIfRateLimited || Airtable.noRetryIfRateLimited || defaultConfig.noRetryIfRateLimited,
+            },
+        });
+
+        this.requestTimeout = opts.requestTimeout || defaultConfig.requestTimeout;
 
         assert(this._apiKey, 'API key is required to connect to Airtable');
     },

test/airtable.test.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var Airtable = require('../lib/airtable');
+
+describe('Airtable', function () {
+  it("doesn't include the API key as an enumerable property", function () {
+    var fakeAirtable = new Airtable({apiKey: 'keyXyz'});
+
+    Object.values(fakeAirtable).forEach(function (value) {
+      expect(value).not.toEqual('keyXyz');
+    });
+  });
+});