Saml2LogoutRequestMixin relayState property should be binding

**Describe the bug**
It seems relayState property should be binding

**To Reproduce**
```
class Saml2LogoutRequestMixin {
..
  @JsonProperty("relayState") Saml2MessageBinding relayState,
```
**Expected behavior**
```
class Saml2LogoutRequestMixin {
..
  @JsonProperty("binding") Saml2MessageBinding binding,
```
