Update invert object utility helper to swap only if value is string

- The helper will no longer throw an error if the object isn't
  flat
- Update utilities tests and remove trivial test cases

Author: remarkablemark

lib/utilities.js

@@ -43,10 +43,9 @@ function invertObject(obj) {
 
     for (key in obj) {
         value = obj[key];
-        if (typeof value === 'object') {
-            throw new Error('`invert`: Object must be flat.');
+        if (typeof value === 'string') {
+            result[value] = key;
         }
-        result[value] = key;
     }
 
     return result;

test/utilities.js

@@ -41,41 +41,29 @@ describe('utilties', function() {
                 invertObject({ foo: 'bar', baz: 'qux' }),
                 { bar: 'foo', qux: 'baz' }
             );
+        });
 
-            // check for unusual cases
+        it('swaps only if value is string', function() {
             assert.deepEqual(
                 invertObject({
                     $: 'dollar',
                     _: 'underscore',
                     num: 1,
-                    u: undefined
+                    u: undefined,
+                    n: null
                 }),
                 {
                     dollar: '$',
-                    underscore: '_',
-                    '1': 'num',
-                    'undefined': 'u'
+                    underscore: '_'
                 }
             );
         });
 
-        it('swaps key with value for array', function() {
-            assert.deepEqual(
-                invertObject(['zero', 'one']),
-                { 'zero': '0', 'one': '1' }
-            );
-        });
-
         it('throws an error if the first argument is invalid', function() {
             [undefined, null, 'foo', 1337].forEach(function(parameter) {
                 assert.throws(function() { invertObject(parameter); });
             });
         })
-
-        it('throws an error if object is not flat', function() {
-            assert.throws(function() { invertObject({ nested: {} }); });
-            assert.throws(function() { invertObject({ obj: null }); });
-        })
     });
 
 });