Enable caching for data loaded by ParsingLoadable

This enables caching manifest files for DASH, HLS and SmoothStreaming.

To disable caching a non cache DataSource should be provided for reading
manifest to the used MediaSource.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183794252

library/core/src/main/java/com/google/android/exoplayer2/upstream/ParsingLoadable.java

@@ -72,7 +72,11 @@ public final class ParsingLoadable<T> implements Loadable {
    * @param parser Parses the object from the response.
    */
   public ParsingLoadable(DataSource dataSource, Uri uri, int type, Parser<? extends T> parser) {
-    this(dataSource, new DataSpec(uri, DataSpec.FLAG_ALLOW_GZIP), type, parser);
+    this(
+        dataSource,
+        new DataSpec(uri, DataSpec.FLAG_ALLOW_GZIP | DataSpec.FLAG_ALLOW_CACHING_UNKNOWN_LENGTH),
+        type,
+        parser);
   }
 
   /**