PSSH: Dump the same version as the loaded data

Currently, even though self.version would be 0, it would dump as a version=1 box with key_IDs set to data (where possible).

This is because pymp4 sets the version to 1 if key_IDs is set with data, as that would make it a v1 PSSH box. So effectively .dump() and .dumps() forces a v1 box as output even if you loaded or created a v0 box.

Fixes #16

pywidevine/pssh.py

@@ -235,7 +235,7 @@ class PSSH:
             version=self.version,
             flags=self.flags,
             system_ID=self.system_id,
-            key_IDs=self.key_ids,
+            key_IDs=self.key_ids if self.version == 1 and self.key_ids else None,
             init_data=self.init_data
         ))