allow forwarding jester.reusePort as not httpbeast.failOnExistingPort

[timotheecour]

depends on dom96/httpbeast#53 which should be merged first

before PR

in terminal tab 1:
nim r --threads jester/tests/example.nim

in terminal tab 2:
nim r --threads jester/tests/example.nim # no failure
curl http://0.0.0.0:5000 # sends cmd to 1st server

after PR

in terminal tab 1:
nim r --threads jester/tests/example.nim

in terminal tab 2:
nim r --threads jester/tests/example.nim
Additional info: address: "" port: 5000 [OSError]