Failing tests for `saveKey` illustrating #29.

Hey I added a failing test for the `saveKey` issue but I'm not sure where the fix goes. I tried adding it in here like so: 
```
diff --git a/lib/base.js b/lib/base.js
index 1e3f781..038f5e1 100644
--- a/lib/base.js
+++ b/lib/base.js
@@ -322,6 +322,9 @@ module.exports = function (mf) {
                     delete out[fields[fidx]];
                 }
             }
+            if (mf.options.saveKey) {
+                out['key'] = 'random key'
+            }
             return out;
         },
```
but it didn't seem to end up in the results. Point me in the right direction @fritzy?