fix adding empty sequence to HTTP headers (#15783)

* fix adding empty sequence to HTTP headers

* add tests

2 files changed, 34 insertions, 4 deletions

diff --git a/lib/pure/httpcore.nim b/lib/pure/httpcore.nim
index af1bfc088..d796c788d 100644
--- a/lib/pure/httpcore.nim
+++ b/lib/pure/httpcore.nim
@@ -166,9 +166,12 @@ proc `[]=`*(headers: HttpHeaders, key, value: string) =
 
 proc `[]=`*(headers: HttpHeaders, key: string, value: seq[string]) =
   ## Sets the header entries associated with ``key`` to the specified list of
-  ## values.
-  ## Replaces any existing values.
-  headers.table[headers.toCaseInsensitive(key)] = value
+  ## values. Replaces any existing values. If ``value`` is empty,
+  ## deletes the header entries associated with ``key``.
+  if value.len > 0:
+    headers.table[headers.toCaseInsensitive(key)] = value
+  else:
+    headers.table.del(headers.toCaseInsensitive(key))
 
 proc add*(headers: HttpHeaders, key, value: string) =
   ## Adds the specified value to the specified key. Appends to any existing
@@ -179,7 +182,7 @@ proc add*(headers: HttpHeaders, key, value: string) =
     headers.table[headers.toCaseInsensitive(key)].add(value)
 
 proc del*(headers: HttpHeaders, key: string) =
-  ## Delete the header entries associated with ``key``
+  ## Deletes the header entries associated with ``key``
   headers.table.del(headers.toCaseInsensitive(key))
 
 iterator pairs*(headers: HttpHeaders): tuple[key, value: string] =
diff --git a/tests/stdlib/thttpcore.nim b/tests/stdlib/thttpcore.nim
index 33c24453e..cb1a0875f 100644
--- a/tests/stdlib/thttpcore.nim
+++ b/tests/stdlib/thttpcore.nim
@@ -51,3 +51,30 @@ suite "httpcore":
     doAssert parseHeader("Accept: foo, bar") == (key: "Accept", value: @["foo", "bar"])
     doAssert parseHeader("Accept: foo, bar, prologue") == (key: "Accept", value: @["foo", "bar", "prologue"])
     doAssert parseHeader("Accept: foo, bar, prologue, starlight") == (key: "Accept", value: @["foo", "bar", "prologue", "starlight"])
+
+  test "add empty sequence to HTTP headers":
+    block:
+      var headers = newHttpHeaders()
+      headers["empty"] = @[]
+
+      doAssert not headers.hasKey("empty")
+
+    block:
+      var headers = newHttpHeaders()
+      headers["existing"] = "true"
+      headers["existing"] = @[]
+
+      doAssert not headers.hasKey("existing")
+
+    block:
+      var headers = newHttpHeaders()
+      headers["existing"] = @["true"]
+      headers["existing"] = @[]
+
+      doAssert not headers.hasKey("existing")
+
+    block:
+      var headers = newHttpHeaders()
+      headers["existing"] = @[]
+      headers["existing"] = @["true"]
+      doAssert headers.hasKey("existing")