fix(fetch): do not set bodyUsed to undefined (nodejs#1346)

Author: tniessen

lib/fetch/body.js

@@ -348,7 +348,7 @@ const properties = {
   bodyUsed: {
     enumerable: true,
     get () {
-      return this[kState].body && util.isDisturbed(this[kState].body.stream)
+      return !!this[kState].body && util.isDisturbed(this[kState].body.stream)
     }
   }
 }

test/node-fetch/response.js

@@ -224,4 +224,9 @@ describe('Response', () => {
     const res = new Response(null, { statusText: undefined })
     expect(res.statusText).to.equal('')
   })
+
+  it('should not set bodyUsed to undefined', () => {
+    const res = new Response()
+    expect(res.bodyUsed).to.be.false
+  })
 })