fixes #22778 regression: contentLength implementation type mismatched (#22780)

fixes #22778
follow up https://github.com/nim-lang/Nim/pull/19835

lib/pure/httpclient.nim

@@ -305,7 +305,7 @@ proc contentLength*(response: Response | AsyncResponse): int =
   ##
   ## A `ValueError` exception will be raised if the value is not an integer.
   ## If the Content-Length header is not set in the response, ContentLength is set to the value -1.
-  var contentLengthHeader = response.headers.getOrDefault("Content-Length", @["-1"])
+  var contentLengthHeader = response.headers.getOrDefault("Content-Length", HttpHeaderValues(@["-1"]))
   result = contentLengthHeader.parseInt()
 
 proc lastModified*(response: Response | AsyncResponse): DateTime =

tests/stdlib/tasynchttpserver.nim

@@ -109,6 +109,7 @@ proc testCustomContentLength() {.async.} =
     doAssert(body == "")
     doAssert(response.headers.hasKey("Content-Length"))
     doAssert(response.headers["Content-Length"] == "0")
+    doAssert contentLength(response) == 0 # bug #22778
 
   runTest(handler, request, test)