fix(client-utils): only call fetch as a function, not a method

[erights]

closes: #XXXX
refs: endojs/endo#31 (comment)

Description

Testing on Brave, the browser's global fetch function works when called with its this bound to either undefined or the browser global object, but does not work if its this is bound to something else. The code this PR fixes was misbehaving because it was calling powers.fetch(...), i.e., happening to call it as a method with its this bound to the irrelevant powers object.

This refactor just expresses the intention of this code, which is just to call the fetch function without passing it some object as its this binding. This seems to fix the problem.

Not at all addressed by this PR: The fact that this problem happened from a programming patterns that seems correct and would have passed review indicates a deeper hazard. This PR does nothing to mitigate this deeper hazard. It only fixes this case where we fell into the hazard.

Security Considerations

none

Scaling Considerations

none

Documentation Considerations

none, except as needed to address the more general hazard.

Testing Considerations

Only tested fetch behavior on Brave. Based on that, proceeding under untested assumption that this refactor will work on all supported browsers.

Upgrade Considerations

none

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