fix: issue#62 strip auth key

Fixing the issue raised in #62

Problem: auth key with leading / trailing whitespaces is throwing exceptions. For example, assigning the secret via `echo` leaves a trailing `\n`.

Solution: strip input auth key as an input sanitization procedure

Notice: Java 8 does not support `String.strip()`, while `String.trim()` does not support Unicode whitespaces. We anticipate ASCII only input and utilizing `trim()` for simplicity.

deepl-java/src/main/java/com/deepl/api/Translator.java

@@ -41,19 +41,24 @@ public class Translator {
    */
   @Deprecated
   public Translator(String authKey, TranslatorOptions options) throws IllegalArgumentException {
-    if (authKey == null || authKey.length() == 0) {
-      throw new IllegalArgumentException("authKey must be a non-empty string");
+    if (authKey == null || authKey.isEmpty()) {
+      throw new IllegalArgumentException("authKey cannot be null or empty");
     }
+
+    String sanitizedAuthKey = authKey.trim();
+
     String serverUrl =
         (options.getServerUrl() != null)
             ? options.getServerUrl()
-            : (isFreeAccountAuthKey(authKey) ? DEEPL_SERVER_URL_FREE : DEEPL_SERVER_URL_PRO);
+            : (isFreeAccountAuthKey(sanitizedAuthKey)
+                ? DEEPL_SERVER_URL_FREE
+                : DEEPL_SERVER_URL_PRO);
 
     Map<String, String> headers = new HashMap<>();
     if (options.getHeaders() != null) {
       headers.putAll(options.getHeaders());
     }
-    headers.putIfAbsent("Authorization", "DeepL-Auth-Key " + authKey);
+    headers.putIfAbsent("Authorization", "DeepL-Auth-Key " + sanitizedAuthKey);
     headers.putIfAbsent(
         "User-Agent",
         constructUserAgentString(options.getSendPlatformInfo(), options.getAppInfo()));

deepl-java/src/test/java/com/deepl/api/GeneralTest.java

@@ -28,6 +28,16 @@ void testEmptyAuthKey() {
             });
   }
 
+  @Test
+  void testNullAuthKey() {
+    IllegalArgumentException thrown =
+        Assertions.assertThrows(
+            IllegalArgumentException.class,
+            () -> {
+              Translator translator = new Translator(null);
+            });
+  }
+
   @Test
   void testInvalidAuthKey() {
     String authKey = "invalid";