Additional integer casting in category querist

[daffidwilde]

Further to #48, we should cast the id column of dimension category queries.

For instance, this example gives us strings, which is inconsistent:

>>> from census21api import CensusAPI
>>> api = CensusAPI()
>>> categories = api.query_categories("HRP", "dimensions", "health_in_general")
>>> categories.dtypes
id                 object
label              object
dimension          object
population_type    object
dtype: object