JS Headers and case-sensitivity

Currently JS Headers such as `Nats-Expected-Last-Sequence` are case-sensitive
in the server to have a fast path rather and avoid `ReadMIMEHeader`.

In this change, we try to keep the fast path but add another check for cases
when the header may have been sent in a different case than expected.

Signed-off-by: Waldemar Quevedo <wally@synadia.com>

server/client.go

@@ -3653,9 +3653,16 @@ func getHeader(key string, hdr []byte) []byte {
 	if len(hdr) == 0 {
 		return nil
 	}
+
+	// Fast-path is for when headers use canonical case.
 	index := bytes.Index(hdr, []byte(key))
 	if index < 0 {
-		return nil
+		// To make JetStream headers case insensitive, do another lookup
+		// with headers in lowercase.
+		index = bytes.Index(hdr, bytes.ToLower([]byte(key)))
+		if index < 0 {
+			return nil
+		}
 	}
 	index += len(key)
 	if index >= len(hdr) {

server/jetstream_test.go

@@ -3427,6 +3427,30 @@ func TestJetStreamPublishExpect(t *testing.T) {
 		t.Fatalf("Expected an error, got %q", resp.Data)
 	}
 
+	// Check with JS Headers in different cases.
+	m = nats.NewMsg("foo.bar")
+	m.Data = []byte("HI")
+	m.Header[JSExpectedLastSeq] = []string{"5"}
+	resp, err = nc.RequestMsg(m, 100*time.Millisecond)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if pa := getPubAckResponse(resp.Data); pa == nil || pa.Error == nil {
+		t.Fatalf("Expected an error, got %q", resp.Data)
+	}
+
+	// Try with JS Header in lower case.
+	// nats-expected-last-sequence: 5
+	m = nats.NewMsg("foo.bar")
+	m.Header[strings.ToLower(JSExpectedLastSeq)] = []string{"5"}
+	resp, err = nc.RequestMsg(m, 100*time.Millisecond)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if pa := getPubAckResponse(resp.Data); pa == nil || pa.Error == nil {
+		t.Fatalf("Expected an error, got %q", resp.Data)
+	}
+
 	// Restart the server and make sure we remember/rebuild last seq and last msgId.
 	// Stop current
 	sd := s.JetStreamConfig().StoreDir