Revert "DateTransform serializes as a number instead of string. The deserializer was already considering this case. Adds millisecond precision to DateTransform"

This reverts commit 47e43938d43cd012c2e89a51c6ea49f74c1c2840.

This change causes errors when server-side handling does not support
numeric values for dates. Example (Rails backend):

```
PG::DatatypeMismatch: ERROR:  column "received_at" is of type timestamp
without time zone but expression is of type bigint
```