issue(SDKFetch.get): 令多个先后不重叠的订阅能共享一个网络请求

避免网络请求的共享随其间订阅数降为 0 （因为订阅之间没有时间重叠，所以
必然存在一个订阅结束之后，订阅数降为 0；后续订阅者进来，订阅数又升至大
于 0）而失效。

另外，由于请求必然会 complete 或者 error，所以不存在资源泄漏的问题。

src/Net/Http.ts

@@ -3,6 +3,7 @@ import 'rxjs/add/observable/dom/ajax'
 import 'rxjs/add/observable/empty'
 import 'rxjs/add/operator/catch'
 import 'rxjs/add/operator/map'
+import 'rxjs/add/operator/publishReplay'
 import { AjaxError } from 'rxjs/observable/dom/AjaxObservable'
 import { Observable } from 'rxjs/Observable'
 import { Observer } from 'rxjs/Observer'

src/SDKFetch.ts

@@ -2,7 +2,7 @@ import 'rxjs/add/observable/defer'
 import 'rxjs/add/observable/throw'
 import 'rxjs/add/operator/catch'
 import 'rxjs/add/operator/map'
-import 'rxjs/add/operator/publishReplay'
+import 'rxjs/add/operator/shareReplay'
 import 'rxjs/add/operator/finally'
 import { Observable } from 'rxjs/Observable'
 import { Http, HttpErrorMessage, HttpResponseWithHeaders, getHttpWithResponseHeaders } from './Net/Http'
@@ -119,8 +119,7 @@ export class SDKFetch {
       const tail = SDKFetch.fetchTail || Date.now()
       const urlWithTail = appendQueryString(urlWithQuery, `_=${ tail }`)
       dist = Observable.defer(() => http.setUrl(urlWithTail).get()['request'])
-        .publishReplay<any>(1)
-        .refCount()
+        .shareReplay<any>(1)
         .finally(() => {
           SDKFetch.FetchStack.delete(urlWithQuery)
         })

test/SDKFetch.spec.ts

@@ -170,7 +170,23 @@ describe('SDKFetch', () => {
       })
   })
 
-  it('shouldn not re-use a matching request A if A is in finished', function* () {
+  it('should re-use a matching request A even if subscriber count has dropped to zero previously', function* () {
+    fetchMock.mock(urlMatcher, Observable.of({ body: { 'A': 'aaaa' } }).delay(10).toPromise())
+
+    const getA = sdkFetch.get(path, { value: 'A' })
+    const getANext = sdkFetch.get(path, { value: 'A' })
+
+    yield Observable.from([ getA, getANext ], Scheduler.asap)
+      .switch() // 确保 getA 的订阅与 getANext 的订阅在时间上是隔离的，
+                // 也就是在 getANext 被订阅前，请求源的订阅数降到了 0
+      .do((response) => {
+        expect(response).to.deep.equal({ 'A': 'aaaa' })
+      })
+
+    expect(fetchMock.calls(urlMatcher).length).to.equal(1)
+  })
+
+  it('should not re-use a matching request A if A is finished', function* () {
     fetchMock.mock(urlMatcher, { body: { 'A': 'aaaa' } })
 
     const getA = sdkFetch.get(path, { value: 'A' })
@@ -180,10 +196,7 @@ describe('SDKFetch', () => {
       .take(3)
       .subscribeOn(Scheduler.asap)
 
-    yield Observable.of(fetchMock.calls(urlMatcher).length)
-      .do((numberOfRequestsReceived) => {
-        expect(numberOfRequestsReceived).to.equal(2)
-      })
+    expect(fetchMock.calls(urlMatcher).length).to.equal(2)
   })
 
   it('should only apply `Http` object `mapFn`(internal-use only) once for each request sent', function* () {

yarn.lock

@@ -2694,9 +2694,9 @@ rxjs-marbles@^2.4.1:
     lodash-es "^4.0.0"
 
 rxjs@^5.4.3:
-  version "5.5.6"
-  resolved "https://registry.npmjs.org/rxjs/-/rxjs-5.5.6.tgz#e31fb96d6fd2ff1fd84bcea8ae9c02d007179c02"
-  integrity sha512-v4Q5HDC0FHAQ7zcBX7T2IL6O5ltl1a2GX4ENjPXg6SjDY69Cmx9v4113C99a4wGF16ClPv5Z8mghuYorVkg/kg==
+  version "5.5.12"
+  resolved "https://registry.npmjs.org/rxjs/-/rxjs-5.5.12.tgz#6fa61b8a77c3d793dbaf270bee2f43f652d741cc"
+  integrity sha512-xx2itnL5sBbqeeiVgNPVuQQ1nC8Jp2WfNJhXWHmElW9YmrpS9UVnNzhP3EH3HFqexO5Tlp8GhYY+WEcqcVMvGw==
   dependencies:
     symbol-observable "1.0.1"