Changed datatype of update_seq and purge_seq to be `number

Overview

The datatype of udpate_seq and purge_seq in actual response object from CouchDB is string but in the nano library it is defined as number. This PR tries to fix this issue. See issue #316 for more details.

Testing recommendations

Run nano against older and newer versions of apache couchdb to validate the datatype in actual response object vs the one defined in nano.d.ts. Also please check if the doc-comments for the fields are correct or not as they are like below:

/** The number of purge operations on the database. */
purge_seq: number | string;

I am not sure if changing the description to be something like The sequence ID of purge operations on the database. i.e. if calling it ID instead of number is appropriate or not.

GitHub issue number

Fixes #316

Checklist

[ x ] Code is written and works correctly;
[ x ] Changes are covered by tests;
[ x ] Documentation reflects the changes;

apache / couchdb-nano