style(service): fix code smells in Context

See https://sonarcloud.io/organizations/gdcc/rules?open=java%3AS3252&rule_key=java%3AS3252

xoai-service-provider/src/main/java/io/gdcc/xoai/serviceprovider/model/Context.java

@@ -22,16 +22,16 @@ public class Context {
     private static final TransformerFactory factory = TransformerFactory.newInstance();
 
     private Transformer transformer;
-    private Map<String, Transformer> metadataTransformers = new HashMap<String, Transformer>();
+    private final Map<String, Transformer> metadataTransformers = new HashMap<>();
     private String baseUrl;
     private Granularity granularity;
     private OAIClient client;
 
     public Context() {
         try {
-            this.withMetadataTransformer("xoai", factory.newTransformer());
+            this.withMetadataTransformer("xoai", Context.factory.newTransformer());
         } catch (TransformerConfigurationException e) {
-            throw new RuntimeException("Unable to initialize identity transformer");
+            throw new IllegalStateException("Unable to initialize identity transformer", e);
         }
     }
 
@@ -95,17 +95,17 @@ public OAIClient getClient () {
     public enum KnownTransformer {
         OAI_DC("to_xoai/oai_dc.xsl");
 
-        private String location;
+        private final String location;
 
         KnownTransformer(String location) {
             this.location = location;
         }
 
         public Transformer transformer () {
             try {
-                return factory.newTransformer(new StreamSource(this.getClass().getClassLoader().getResourceAsStream(location)));
+                return Context.factory.newTransformer(new StreamSource(this.getClass().getClassLoader().getResourceAsStream(location)));
             } catch (TransformerConfigurationException e) {
-                throw new RuntimeException("Unable to load resource file '"+location+"'", e);
+                throw new IllegalStateException("Unable to load resource file '" + location + "'", e);
             }
         }
     }