Add default User-Agent header to HTTP requests on Android (#2278)

* Add a default User-Agent header in DefaultHttpHandler

* Clean up raster-simple demo scene

platforms/android/tangram/src/main/java/com/mapzen/tangram/networking/DefaultHttpHandler.java

@@ -98,6 +98,9 @@ public void onResponse(@NonNull final Call call, final Response response) {
             }
         };
         final Request.Builder builder = new Request.Builder().url(httpUrl);
+        // Some tile endpoints treat requests as "unauthorized" if they have no User-Agent header,
+        // so add a default value to every request. Users can override this in configureRequest().
+        builder.addHeader("User-Agent", "tangram");
         configureRequest(httpUrl, builder);
         final Request request = builder.build();
         Call call = okClient.newCall(request);

scenes/raster-simple.yaml

@@ -2,16 +2,15 @@ global:
     mapzen_api_key: ''
 
 sources:
-    stamen-terrain:
+    osm-raster:
         type: Raster
-        #url: http://a.tile.stamen.com/terrain-background/{z}/{x}/{y}.jpg
-        url: http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png
+        url: https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png
         url_subdomains: [a, b, c]
 
 layers:
     terrain:
-        data: { source: stamen-terrain }
+        data: 
+            source: osm-raster
         draw:
             raster:
-                #color: [0.8, 0.8, 0.8]
                 order: 0 # draw on bottom