fix(toJSON): the json() method should be called once per response

src/toJSON.js

@@ -4,6 +4,8 @@ const Rx = require('rx')
 const targ = require('argtoob')
 
 module.exports = response => {
-  const json = response.flatMap(x => x.json())
-  return Rx.Observable.zip(json, response, targ('json', 'response'))
+  const json = response.flatMap(x => x.json()).publish()
+  json.connect()
+  const zip = Rx.Observable.zip(json, response, targ('json', 'response'))
+  return zip
 }

test/test.toJSON.js

@@ -3,15 +3,15 @@
  */
 
 'use strict'
-
+import {spy} from 'sinon'
 import {TestScheduler, ReactiveTest} from 'rx'
 import test from 'ava'
 import toJSON from '../src/toJSON'
 
 const {onNext} = ReactiveTest
 const createResponse = val => ({
   status: 200,
-  json: () => [val + '-json']
+  json: spy(() => [val + '-json'])
 })
 
 test(t => {
@@ -30,3 +30,16 @@ test(t => {
     onNext(230, {json: '2-json', response: mocks[2]})
   ])
 })
+
+test('json():callCount', t => {
+  const sh = new TestScheduler()
+  const mockResponse = createResponse(0)
+  const ob0 = sh.createObserver()
+  const ob1 = sh.createObserver()
+  const responses = sh.createHotObservable(onNext(210, mockResponse))
+  const json = toJSON(responses)
+  sh.scheduleAbsolute(null, 200, () => json.subscribe(ob0))
+  sh.scheduleAbsolute(null, 205, () => json.subscribe(ob1))
+  sh.start()
+  t.is(mockResponse.json.callCount, 1)
+})