Addition: Docs need to say that _refresh_states() also takes a non iterable

[IAlwaysBeCoding]

The docs need to be changed to let the user know that _refresh_states() also takes a non iterable, aka a single object not contained in a list.

frontera/frontera/strategy/__init__.py

Lines 139 to 145 in 8ac2316

	def refresh_states(self, requests):
	"""
	Retrieves states for all requests from storage.
	:param requests: list(:class:`Request <frontera.core.models.Request>`)
	"""
	self._states_context.refresh_and_keep(requests)

That's because the function refresh_and_keep() converts the non iterable object into a non-iterable object contained within a list.

frontera/frontera/core/manager.py

Lines 818 to 822 in 8ac2316

	def refresh_and_keep(self, requests):
	self.to_fetch(requests)
	self.fetch()
	self.states.set_states(requests)
	self._requests.extend(requests if isinstance(requests, Iterable) else [requests])

[sibiryakov]

Good catch! Can you make PR @IAlwaysBeCoding ?