rx-gorm-rest-client has no Content-Length headers, interceptors can't add them

When using the RxGORM REST client, certain requests have no Content-Length headers set, which is required by a number of API's. They cannot be calculated from inside a RequestBuilderInterceptor since it has no access to the JSON string or HTTP body.

Start by tracing the RxGORM REST client request construction and the RequestBuilderInterceptor entry point named in the issue. Determine where the JSON string or HTTP body becomes available, then verify that affected requests include Content-Length and that interceptors can work with it.

Written by the indexing model from the issue text.