astroquery.cds.find_datasets result should respect the order of requested fields

When asking for a specific set of fields, the resulting Astropy table has a different ordering of the columns.

Example:

```python
ds = cds.find_datasets(meta_data ="*", fields=['ID', 'moc_access_url', 'hips_creation_date', 'hips_creator'])
ds.colnames
```
returns `['hips_creator', 'ID', 'moc_access_url', 'hips_creation_date']` whereas I was expecting to get the same order as what I asked in the fields parameter.

@bmatthieu3 : could you have a look?

Start at the astroquery.cds.find_datasets entry point and reproduce the reported call with the requested fields. Trace how the Astropy table columns are assembled, then verify completion by confirming that ds.colnames exactly matches the order supplied in fields.

Written by the indexing model from the issue text.