fix bug with config service interpreting rare client secret strings as exponential numbers

closes #5815
- replaces isNaN function call with isFinite in config service
- adds config service unit test

ghost/admin/app/services/config.js

@@ -1,7 +1,7 @@
 import Ember from 'ember';
 
 function isNumeric(num) {
-    return !isNaN(num);
+    return Ember.$.isNumeric(num);
 }
 
 function _mapType(val) {

ghost/admin/tests/unit/services/config-test.js

@@ -0,0 +1,34 @@
+/* jshint expr:true */
+import { expect } from 'chai';
+import {
+    describeModule,
+    it
+} from 'ember-mocha';
+
+import Ember from 'ember';
+
+describeModule(
+    'service:config',
+    'ConfigService',
+    {
+        // Specify the other units that are required for this test.
+        // needs: ['service:foo']
+    },
+    function () {
+        // Replace this with your real tests.
+        it('exists', function () {
+            var service = this.subject();
+            expect(service).to.be.ok;
+        });
+
+        it('correctly parses a client secret', function () {
+            Ember.$('<meta>').attr('name', 'env-clientSecret')
+                .attr('content', '23e435234423')
+                .appendTo('head');
+
+            var service = this.subject();
+
+            expect(service.get('clientSecret')).to.equal('23e435234423');
+        });
+    }
+);