Fixes #16436 (#16695)

* Fixes #16436

* Comments addressed

tests/stdlib/thttpclient_standalone.nim

@@ -0,0 +1,23 @@
+discard """
+  cmd: "nim c --threads:on $file"
+"""
+
+import asynchttpserver, httpclient, asyncdispatch
+
+block: # bug #16436
+  proc startServer() {.async.} =
+    proc cb(req: Request) {.async.} =
+      let headers = { "Content-length": "15"} # Provide invalid content-length
+      await req.respond(Http200, "Hello World", headers.newHttpHeaders())
+
+    var server = newAsyncHttpServer()
+    await server.serve(Port(5555), cb)
+
+  proc runClient() {.async.} =
+    let c = newAsyncHttpClient(headers = {"Connection": "close"}.newHttpHeaders)
+    let r = await c.getContent("http://127.0.0.1:5555")
+    doAssert false, "should fail earlier"
+
+  asyncCheck startServer()
+  doAssertRaises(ProtocolError):
+    waitFor runClient()