Fix missing set-cookie issue.

It appears newer versions of FreeIPA (>4.8.6) do not return the set-cookie
header in every response. It's only included in the initial login response.
Here we were updating the sessionID on every rpc call and erroring out if the
set-cookie header was missing. As this is not necessary we simply return if the
header is missing.

go.mod

@@ -1,5 +1,7 @@
 module github.com/ubccr/goipa
 
+go 1.14
+
 require (
 	github.com/go-ini/ini v1.37.0
 	github.com/hashicorp/go-uuid v0.0.0-20180228145832-27454136f036 // indirect

ipa.go

@@ -340,7 +340,7 @@ func (c *Client) setSessionID(res *http.Response) error {
 
 	cookie := res.Header.Get("Set-Cookie")
 	if len(cookie) == 0 {
-		return errors.New("empty set-cookie header")
+		return nil
 	}
 
 	ipaSession := ""