Reported issues with Shielding

[TartanLlama]

Some customers have reported issues with shielding and the JS SDK, particularly seeing "Requested backend named name followed by random characters" in errors. #1429 was suggested by a customer and implemented to fix an obvious bug, but we should ensure that this fixes reported issues with shielding and find a reproduction that we can add to the test suite.

[cupofjoakim]

@TartanLlama I believe my company reported this, and just to help a bit, here are some example errors.

(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**om/paymen0' does not exist", "__fastly_helpers.js", 6882))
(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**ots.txt' does not exist", "__fastly_helpers.js", 6882))
(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**/' does not exist", "__fastly_helpers.js", 6882))
(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**om/' does not exist", "__fastly_helpers.js", 6882))
(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**com/wp-js0' does not exist", "__fastly_helpers.js", 6882))
(new TypeError("Requested backend named '**fastly-shield-INTER-PAR-SSL**icon.ico' does not exist", "__fastly_helpers.js", 6882))

It does seem that the backend name is suffixed with parts of the requested url. Whats weird is that I did not experience this on every request, but that might have something to do with the suffix being of uneven lengths.