fix dead-url in writer.go

[xulike666]

I couldn't find the header format for a series of GELF chunked messages.
Signed-off-by: Aaron.L.Xu likexu@harmonycloud.cn

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Aaron.L.Xu seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.

[xulike666]

I have signed the CLA for Graylog2/go-gelf.

[joschi]

Thanks!