fix: add default value to Form[decorators], correct FormSpy test

Author: mrfratello

src/FormSpy.test.js

@@ -341,7 +341,7 @@ describe('FormSpy', () => {
             <FormSpy
               subscription={{ dirty: true, values: true }}
               onChange={spy}
-              render={render}
+              render={renderSpy}
             />
           </form>
         )}

src/ReactFinalForm.js

@@ -42,7 +42,7 @@ export const all: FormSubscription = formSubscriptionItems.reduce(
 
 function ReactFinalForm<FormValues: FormValuesShape>({
   debug,
-  decorators,
+  decorators = [],
   destroyOnUnregister,
   form: alternateFormApi,
   initialValues,