SDK3: Support subdoc contentAs(idx, String.class)

For txns, I need to retrieve a subdoc field as a String.  But I get this error on doing contentAs(idx, String.class):

com.couchbase.client.core.deps.com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_OBJECT token

 at [Source: (byte[])"{"val":1}"; line: 1, column: 1] 

I can contentAs(idx, JsonObject.class) fine, and then convert it into a String, but it would be nice to avoid this unnecessary step.

This worked fine in SDK2.