Add unit tests for secret metadata models and fixed JSON property name

src/main/java/de/stklcode/jvault/connector/model/response/MetadataResponse.java

@@ -49,7 +49,8 @@ public class MetadataResponse extends VaultDataResponse {
 
     /**
      * Get the actual metadata.
-     * @return
+     *
+     * @return Metadata.
      */
     public SecretMetadata getMetadata() {
         return metadata;

src/main/java/de/stklcode/jvault/connector/model/response/embedded/SecretMetadata.java

@@ -40,7 +40,7 @@ public final class SecretMetadata {
     @JsonProperty("current_version")
     private Integer currentVersion;
 
-    @JsonProperty("max_version")
+    @JsonProperty("max_versions")
     private Integer maxVersions;
 
     @JsonProperty("oldest_version")