gofiber adapter, fix injecting header logic

[drakejin]

Issue #, if available:

88

• this PR is duplicated(https://github.com/awslabs/aws-lambda-go-api-proxy/pull/89), I carry on the issues to this PR.

Description of changes:

1. Add Test Cases

   you can see fiber/fiberlambda_test.go

2. fix Injecting logic to request header

    switch key {
    // NOTI: fiber.HeaderTransferEncoding is no need to add case statements
    case fiber.HeaderHost,
        fiber.HeaderContentType,
        fiber.HeaderUserAgent,
        fiber.HeaderContentLength,
        fiber.HeaderConnection:
        req.Header.Set(key, v)
    default:
        req.Header.Add(key, v)
    }

3. fix Injecting logic to response header
   • references: https://github.com/gofiber/adaptor/issues/64

     fctx.Response.Header.VisitAll(func(k, v []byte) {
     w.Header().Add(utils.UnsafeString(k), utils.UnsafeString(v))
     })

Mention for noti

@ReneWerner87

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[drakejin]

Hi -! @sapessi

~If you feel free, let me know how should I do to pass this Travis CI... Lots of gofiber fans are waiting for this PR.~

~I can just see this below message on TravisCI (Actually, I had not used travisCI ever)~

cc @ThalysonR

[drakejin]

@sapessi Hi. Would you check this PR? I want to know when you can merge this PR. If you are okay, let me know what I should do to merge this?

[sapessi]

Apologies folks. Merging soon.