Fixes a parsing error introduced by an upstream fix in cookiejar: bmeck/node-cookiejar#38

Prior to 2.1.3, cookiejar was incorrectly producing cookie headers containing multiple cookies with no spaces after the separating semicolon. That space is explicitly required in the RFC.

When Supertest merges cookies provided with via .set() with cookies from its internal cookiejar instance, it assumes that incorrect, spaceless format. This results in any cookie names after the first in a given header erroneously containing leading spaces.

I added a test for cookie validity and the fix to split on ; instead of just ;.

Found this issue the same way as #1761, but it's actually unrelated. Fun! 😄

Checklist

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