Fix/rules parsing

[jzellman]

2 commits:

fix: use constructed JSON parser on rules responses to properly parse ids

If the request library is provided a JSON body on POST, it will
automatically parse the response using JSON.parse. However, this
will incorrectly decode the id field. This change uses parser.parse
(defaults to JSONBigInt) to ensure that the id is properly decoded.

refactor: pull up headers to an instance variable