Refactored extraction of auth backends

Changed iteration over keys ot iteration over entries.
Implemented unit test as part of this process.

src/main/java/de/stklcode/jvault/connector/model/AuthBackend.java

@@ -27,6 +27,7 @@ public enum AuthBackend {
     APPID("app-id"),
     APPROLE("approle"),
     USERPASS("userpass"),
+    GITHUB("github"),   // Not supported yet.
     UNKNOWN("");
 
     private final String type;

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

@@ -45,11 +45,10 @@ public final class AuthMethodsResponse extends VaultDataResponse {
     @Override
     public void setData(final Map<String, Object> data) throws InvalidResponseException {
         ObjectMapper mapper = new ObjectMapper();
-        for (String path : data.keySet()) {
+        for (Map.Entry<String, Object> entry : data.entrySet()) {
             try {
-                this.supportedMethods.put(
-                        path, mapper.readValue(mapper.writeValueAsString(data.get(path)),
-                        AuthMethod.class));
+                this.supportedMethods.put(entry.getKey(),
+                        mapper.readValue(mapper.writeValueAsString(entry.getValue()), AuthMethod.class));
             } catch (IOException e) {
                 throw new InvalidResponseException();
             }