After submitting a form the isSubmitting property is automatically set back to false without calling setSubmitting()

## 🐛 Bug report

### Current Behavior

After submitting a form the isSubmitting property is automatically set back to false without calling setSubmitting(). Note how fields and buttons are disabled after clicking submit, but then become reenabled.

### Expected behavior

isSubmitting should remain true until developer calls `setSubmitting(true)`

### Your environment

| Software | Version(s) |
| ---------------- | ---------- |
| Formik | 2.0.3
| React | 16.11.0
| Browser | Chrome
| npm/Yarn | NPM
| Operating System | Windows

No source file or test is named. Start by reproducing the submission behavior with Formik 2.0.3 and React 16.11.0, then trace the submission state transition; done means isSubmitting stays true until the developer calls setSubmitting().

Written by the indexing model from the issue text.