Add a default user agent in OkHttp integration.

Fixes #436.

integration/okhttp/src/main/java/com/bumptech/glide/integration/okhttp/OkHttpStreamFetcher.java

@@ -17,6 +17,8 @@ import java.util.Map;
  * Fetches an {@link InputStream} using the okhttp library.
  */
 public class OkHttpStreamFetcher implements DataFetcher<InputStream> {
+    private static final String USER_AGENT_HEADER = "User-Agent";
+    private static final String DEFAULT_USER_AGENT = System.getProperty("http.agent");
     private final OkHttpClient client;
     private final GlideUrl url;
     private InputStream stream;
@@ -31,8 +33,15 @@ public class OkHttpStreamFetcher implements DataFetcher<InputStream> {
     public InputStream loadData(Priority priority) throws Exception {
         Request.Builder requestBuilder = new Request.Builder()
                 .url(url.toStringUrl());
+
+        boolean isUserAgentSet = false;
         for (Map.Entry<String, String> headerEntry : url.getHeaders().entrySet()) {
-            requestBuilder.addHeader(headerEntry.getKey(), headerEntry.getValue());
+            String key = headerEntry.getKey();
+            requestBuilder.addHeader(key, headerEntry.getValue());
+            isUserAgentSet |= USER_AGENT_HEADER.equalsIgnoreCase(key);
+        }
+        if (!isUserAgentSet) {
+            requestBuilder.addHeader(USER_AGENT_HEADER, DEFAULT_USER_AGENT);
         }
         Request request = requestBuilder.build();