Filters.regexp not working but DBQuery.regexp is fine

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I've recently migrated from 2.10.0 to 4.2.1 and removed all deprecated code.

I'm using JacksonMongoCollection and almost everything works great except a corner case: for Map-based embedded docs: Filters.regexp does not work where DBQuery.regexp is fine.

• Filters.regexp("field", Pattern.compile("^[0-9]")) works fine (serialized to a regexp doc)
• Filters.regexp("field.subfield", Pattern.compile("^[0-9]")) is NOT working (serialized to a string through a ToStringSerializer) (field being a generic Map<String,Object> in java)

Using a custom ObjectMapper and a custom PatternSerializer just inheriting EmbeddedObjectSerializer fixes the issue, but this should not fail to begin with.

I've prepared a PR for this, but I'm unsure if it's the best way to fix it, but serializing and deserializing in DocumentSerializationUtilsImpl#serializeFilter seems to lose information as I get at runtime a MapSerializer then ToStringSerializer and the db query sent to the server does not work.