HivePartitioningOptions does not convert the `fields` field

#### Environment details

1. Specify the API at the beginning of the title. For example, "BigQuery: ..."). BigQuery:Core
2. OS type and version: All
3. Java version: All
4.  version(s): 0.25.1

#### Steps to reproduce

  1. Call getTable() on a Hive partitioned table
  2. Check the HivePartitioningOptions
