Feature suggestion: allow new headers to be passed to PyPortal.fetch, just as a new URL can be

PyPortal.fetch can be used to get data from different URLs simply by passing a new URL when it is called. However this cannot be done for headers. headers cannot currently be passed in a call to that function. Instead is uses the information, if any, set when the instance of PyPortal was first initialized.

I suggest adding the ability to pass a new header as a named argument in the call, so that different URL's with different headers can be called more easily. I think this would be an easy change. If the parameter is passed in, then it just replaces _headers in the network call.

Start by locating the PyPortal.fetch implementation and inspect how it obtains headers from the initialized instance. Done means fetch accepts an optional named headers argument for a call, uses it when supplied, and retains the existing behavior when it is omitted; verify calls using different URLs and headers.

Written by the indexing model from the issue text.