Fix serialization of 'datetime' objects in '--write-metadata'

Simplified universal serialization support in json.dump() can be achieved
by passing 'default=str', which was already the case in DataJob.run()
for -j/--dump-json, but not for the 'metadata' post-processor.

This commit introduces util.dump_json() that (more or less) unifies the
JSON output procedure of both --write-metadata and --dump-json.

(#251, #252)

gallery_dl/postprocessor/metadata.py

@@ -10,7 +10,6 @@
 
 from .common import PostProcessor
 from .. import util
-import json
 
 
 class MetadataPP(PostProcessor):
@@ -61,13 +60,7 @@ class MetadataPP(PostProcessor):
         file.write("\n")
 
     def _write_json(self, file, pathfmt):
-        json.dump(
-            pathfmt.keywords,
-            file,
-            sort_keys=True,
-            indent=self.indent,
-            ensure_ascii=self.ascii,
-        )
+        util.dump_json(pathfmt.keywords, file, self.ascii, self.indent)
 
 
 __postprocessor__ = MetadataPP