Silence hint:performance message when using very basic http client (#23832)

code such as:
```Nim
import std/httpclient # nim c --hint:performance:on
echo newHttpClient(proxy=nil,
  headers=newHttpHeaders({"Accept": "*/*"})).getContent("x")
```
(Fix was suggested by @ringabout in a private channel.)

Seems useful since `httpclient` is so basic/probably pervasive with many
hundreds of `import`s across the NimbleVerse.

lib/pure/httpcore.nim

@@ -164,7 +164,8 @@ func `[]`*(headers: HttpHeaders, key: string): HttpHeaderValues =
   ## To access multiple values of a key, use the overloaded `[]` below or
   ## to get all of them access the `table` field directly.
   {.cast(noSideEffect).}:
-    return headers.table[headers.toCaseInsensitive(key)].HttpHeaderValues
+    let tmp = headers.table[headers.toCaseInsensitive(key)]
+    return HttpHeaderValues(tmp)
 
 converter toString*(values: HttpHeaderValues): string =
   return seq[string](values)[0]