fix: error being thrown with Vue.extend when functional (#359)

* Hotfix: Fix error being thrown with `Vue.extend` when functional

If using `functional` component with `Vue.extend` an error is thrown,
this is avoided by checking the options object

* test: remove .only

Author: blake-newman

src/lib/create-instance.js

@@ -27,7 +27,7 @@ export default function createConstructor (
     addMocks(mountingOptions.mocks, vue)
   }
 
-  if (component.functional) {
+  if ((component.options && component.options.functional) || component.functional) {
     component = createFunctionalComponent(component, mountingOptions)
   } else if (mountingOptions.context) {
     throwError(

test/unit/specs/mount/options/context.spec.js

@@ -1,3 +1,4 @@
+import Vue from 'vue'
 import { mount } from '~vue-test-utils'
 import { vueVersion } from '~resources/test-utils'
 
@@ -34,6 +35,16 @@ describe('context', () => {
     expect(fn).to.throw().with.property('message', message)
   })
 
+  it('does not throw error if functional component with Vue.extend', () => {
+    const Component = Vue.extend({
+      functional: true,
+      render: h => h('div')
+    })
+    const context = {}
+    const fn = () => mount(Component, { context })
+    expect(fn).not.to.throw()
+  })
+
   it('throws error if context option is not an object', () => {
     const Component = {
       functional: true,