New runnableExample for newAsyncHttpClient() (#14045)

lib/pure/httpclient.nim

@@ -573,6 +573,17 @@ proc newHttpClient*(userAgent = defUserAgent, maxRedirects = 5,
   ## ``TimeoutError`` is raised.
   ##
   ## ``headers`` specifies the HTTP Headers.
+  runnableExamples:
+    import asyncdispatch, httpclient, strutils
+
+    proc asyncProc(): Future[string] {.async.} =
+      var client = newAsyncHttpClient()
+      return await client.getContent("http://example.com")
+
+    let exampleHtml = waitFor asyncProc()
+    assert "Example Domain" in exampleHtml
+    assert not ("Pizza" in exampleHtml)
+
   new result
   result.headers = headers
   result.userAgent = userAgent