Fix date filter over JSON-RPC: send Time/Date as ISO 8601 string

YAML parses date strings as Time objects. Time responds to to_liquid,
so DropProxy.wrap was wrapping it as an RPC drop. The server's
RpcDropProxy is not a Time, so the date filter couldn't format it.

Fix: add Time/Date/DateTime cases to wrap that send iso8601 string.
The date filter parses the string on the server side.

JSON-RPC: 4286 passes, 6 failures (down from 9), 992 skipped.
Remaining 6: 4 filesystem error messages, 2 resource limits.

Author: tobi

lib/liquid/spec/json_rpc/drop_proxy.rb

@@ -72,6 +72,12 @@ def wrap(obj, registry, seen = {}.compare_by_identity)
               else
                 obj
               end
+            when Time
+              # Time is a valid Liquid value but can't be JSON-encoded.
+              # Send as ISO 8601 string — the date filter can parse it.
+              obj.iso8601
+            when Date, DateTime
+              obj.iso8601
             when Symbol
               # Symbols can't be faithfully represented in JSON.
               # Send a _ruby_type marker so the server can optionally