Change user agent to "openHAB client for Android" (#945)

* Change user agent to "openHAB client for Android"

Signed-off-by: mueller-ma <mueller-ma@users.noreply.github.com>

* Add test for HttpClient.buildUrl() when relative url starts with slash

Signed-off-by: mueller-ma <mueller-ma@users.noreply.github.com>

mobile/src/main/java/org/openhab/habdroid/util/HttpClient.java

@@ -63,6 +63,7 @@ public abstract class HttpClient {
             long timeoutMillis, CachingMode caching) {
         Request.Builder requestBuilder = new Request.Builder();
         requestBuilder.url(buildUrl(url));
+        requestBuilder.addHeader("User-Agent", "openHAB client for Android");
         if (mAuthHeader != null) {
             requestBuilder.addHeader("Authorization", mAuthHeader);
         }

mobile/src/test/java/org/openhab/habdroid/core/connection/DefaultConnectionTest.java

@@ -130,6 +130,10 @@ public class DefaultConnectionTest {
         assertFalse("The request should never succeed in tests", result.isSuccessful());
         assertEquals(TEST_BASE_URL + "/rest/test", result.request.url().toString());
         result.close();
+
+        result = testConnection.getSyncHttpClient().get("/rest/test");
+        assertEquals(TEST_BASE_URL + "/rest/test", result.request.url().toString());
+        result.close();
     }
 
     @Test