Do not send NameIDPolicy with a blank Format (#53)

* Do not send NameIDPolicy with a blank Format

Previously, if the `name_id_format` function returned None, this would
cause `NameIdPolicy` to have None as a format, leading to the following
tag in an AuthnRequest:

```xml
<saml2p:NameIDPolicy AllowCreate="true"/>
```

Skip creating a `Some(NameIDPolicy)` if `name_id_format` returns `None`.

Additionally, do not override if `authn_name_id_format` is set to
`Unspecified`: previously this would cause `name_id_format` to return
`None` where a caller explicitly set it to `Unspecified`.

* fmt

src/service_provider/mod.rs

@@ -236,11 +236,8 @@ impl ServiceProvider {
         self.authn_name_id_format
             .clone()
             .and_then(|v| -> Option<String> {
-                let unspecified = NameIdFormat::UnspecifiedNameIDFormat.value();
                 if v.is_empty() {
                     Some(NameIdFormat::TransientNameIDFormat.value().to_string())
-                } else if v == unspecified {
-                    None
                 } else {
                     Some(v)
                 }
@@ -478,9 +475,9 @@ impl ServiceProvider {
                 value: entity_id,
                 ..Issuer::default()
             }),
-            name_id_policy: Some(NameIdPolicy {
+            name_id_policy: self.name_id_format().map(|format| NameIdPolicy {
                 allow_create: Some(true),
-                format: self.name_id_format(),
+                format: Some(format),
                 ..NameIdPolicy::default()
             }),
             force_authn: Some(self.force_authn),