Fix #13573 and #13574 (#13575)

* Fix https://github.com/nim-lang/Nim/issues/13573 and https://github.com/nim-lang/Nim/issues/13574

* Restored asynchttpserver

lib/pure/httpcore.nim

@@ -105,11 +105,15 @@ proc newHttpHeaders*(): HttpHeaders =
 
 proc newHttpHeaders*(keyValuePairs:
     openArray[tuple[key: string, val: string]]): HttpHeaders =
-  var pairs: seq[tuple[key: string, val: seq[string]]] = @[]
-  for pair in keyValuePairs:
-    pairs.add((pair.key.toLowerAscii(), @[pair.val]))
   new result
-  result.table = newTable[string, seq[string]](pairs)
+  result.table = newTable[string, seq[string]]()
+  for pair in keyValuePairs:
+    let key = pair.key.toLowerAscii()
+    if key in result.table:
+      result.table[key].add(pair.val)
+    else:
+      result.table[key] = @[pair.val]
+
 
 proc `$`*(headers: HttpHeaders): string =
   return $headers.table

tests/stdlib/thttpcore.nim

@@ -4,7 +4,7 @@ output: "[Suite] httpcore"
 
 import unittest
 
-import httpcore
+import httpcore, strutils
 
 suite "httpcore":
 
@@ -29,3 +29,8 @@ suite "httpcore":
     assert "baR" in h["cookiE"]
     h.del("coOKie")
     assert h.len == 0
+
+    # Test that header constructor works with repeated values
+    let h1 = newHttpHeaders({"a": "1", "a": "2", "A": "3"})
+
+    assert seq[string](h1["a"]).join(",") == "1,2,3"