Add barebones asynchttpserver tests (#13883)

* Add isNil check to custom Content-Length.

Related to #13866.

* Setup barebones asynchttpserver tests.

tests/stdlib/tasynchttpserver.nim

@@ -0,0 +1,119 @@
+discard """
+  cmd: "nim c --threads:on $file"
+  exitcode: 0
+  output: "OK"
+  disabled: false
+"""
+
+import strutils
+from net import TimeoutError
+
+import httpclient, asynchttpserver, asyncdispatch, asyncfutures
+
+template runTest(
+    handler: proc (request: Request): Future[void] {.gcsafe.},
+    request: proc (server: AsyncHttpServer): Future[AsyncResponse],
+    test: proc (response: AsyncResponse, body: string): Future[void]) =
+
+  let server = newAsyncHttpServer()
+
+  discard server.serve(Port(64123), handler)
+
+  let
+    response = waitFor(request(server))
+    body = waitFor(response.body)
+
+  discard test(response, body)
+
+proc test200() {.async.} =
+  proc handler(request: Request) {.async.} =
+    await request.respond(Http200, "Hello World, 200")
+
+  proc request(server: AsyncHttpServer): Future[AsyncResponse] {.async.} =
+    let
+      client = newAsyncHttpClient()
+      clientResponse = await client.request("http://localhost:64123/")
+
+    server.close()
+
+    return clientResponse
+
+  proc test(response: AsyncResponse, body: string) {.async.} =
+    doAssert(response.status == Http200)
+    doAssert(body == "Hello World, 200")
+    doAssert(response.headers.hasKey("Content-Length"))
+    doAssert(response.headers["Content-Length"] == "16")
+
+  runTest(handler, request, test)
+
+proc test404() {.async.} =
+  proc handler(request: Request) {.async.} =
+    await request.respond(Http404, "Hello World, 404")
+
+  proc request(server: AsyncHttpServer): Future[AsyncResponse] {.async.} =
+    let
+      client = newAsyncHttpClient()
+      clientResponse = await client.request("http://localhost:64123/")
+
+    server.close()
+
+    return clientResponse
+
+  proc test(response: AsyncResponse, body: string) {.async.} =
+    doAssert(response.status == Http404)
+    doAssert(body == "Hello World, 404")
+    doAssert(response.headers.hasKey("Content-Length"))
+    doAssert(response.headers["Content-Length"] == "16")
+
+  runTest(handler, request, test)
+
+proc testCustomEmptyHeaders() {.async.} =
+  proc handler(request: Request) {.async.} =
+    await request.respond(Http200, "Hello World, 200", newHttpHeaders())
+
+  proc request(server: AsyncHttpServer): Future[AsyncResponse] {.async.} =
+    let
+      client = newAsyncHttpClient()
+      clientResponse = await client.request("http://localhost:64123/")
+
+    server.close()
+
+    return clientResponse
+
+  proc test(response: AsyncResponse, body: string) {.async.} =
+    doAssert(response.status == Http200)
+    doAssert(body == "Hello World, 200")
+    doAssert(response.headers.hasKey("Content-Length"))
+    doAssert(response.headers["Content-Length"] == "16")
+
+  runTest(handler, request, test)
+
+proc testCustomContentLength() {.async.} =
+  proc handler(request: Request) {.async.} =
+    let headers = newHttpHeaders()
+    headers["Content-Length"] = "0"
+    await request.respond(Http200, "Hello World, 200", headers)
+
+  proc request(server: AsyncHttpServer): Future[AsyncResponse] {.async.} =
+    let
+      client = newAsyncHttpClient()
+      clientResponse = await client.request("http://localhost:64123/")
+
+    server.close()
+
+    return clientResponse
+
+  proc test(response: AsyncResponse, body: string) {.async.} =
+    doAssert(response.status == Http200)
+    doAssert(body == "")
+    doAssert(response.headers.hasKey("Content-Length"))
+    doAssert(response.headers["Content-Length"] == "0")
+
+  runTest(handler, request, test)
+
+waitfor(test200())
+waitfor(test404())
+waitfor(testCustomEmptyHeaders())
+waitfor(testCustomContentLength())
+
+echo "OK"