Prerender Fargate Leading Slash Logic

[finnholland]

Description of the proposed changes

• A change to prerender's logic when it comes to adding a correct leading slash onto the location returned in 301 redirect responses. This affected some prerender instances and led to redirected URLs having doubly-appended responses for example: https://example.com/trains/steam-engines/trains/steam-engines rather than the correct response https://example.com/trains/steam-engines
• Changed the logic to be a little more clear

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[aaronmedina-dev]

LGTM