fix 648 url object

[berndfuhrmann]

Issue #, if available:
#648
Description of changes:
Implementation:
args[0] in fetch_p.js, line 74 could be any stringifyable object, not just a string. That object should be converted to a string and used as URL. The only exception to this is if args[0] is already a request object.
This PR assumes that such a request object must be from the same implementation as the one given by requestClass. So if you would put a one Request object from e.g. node-fetch into the native fetch implementation, this will stringify that request object.
I couldn't find any "official" way to distinguish between a request object and a stringifyable object. But you're supposed to call fetch with the corresponding request implementation, I think this is fair game.

For that reason, the test was changed, so it will provide the native Request object.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.56%. Comparing base (3b89008) to head (96043e3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #697   +/-   ##
=======================================
  Coverage   83.56%   83.56%           
=======================================
  Files          36       36           
  Lines        1813     1813           
=======================================
  Hits         1515     1515           
  Misses        298      298           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.