DB client should set the `Accept` header on requests

[Mr0grog]

Technically, the web-monitoring-db client in db.py should set the HTTP Accept header to application/json when making requests.

Most of the time it’s ok not to set that header because the server will almost always return JSON regardless of what you asked for, but not always! In edgi-govdata-archiving/web-monitoring-db#614, we’ll be using that header to determine what to do in dev mode, which will affect what type of exception you get when a request fails.

[Mr0grog]

Right now, each method on db.Client is responsible for making its own HTTP request. For example:

web-monitoring-processing/web_monitoring/db.py

Lines 254 to 269 in bf1743d

	def get_page(self, page_id):
	"""
	Lookup a specific Page by ID.
	Parameters
	----------
	page_id : string
	Returns
	-------
	response : dict
	"""
	url = f'{self._api_url}/pages/{page_id}'
	res = requests.get(url, auth=self._auth)
	_process_errors(res)
	result = res.json()

We should probably pull the request, error handling, and JSON parsing into a separate private method. Then make sure that method sets the Accept header for every request.