Set the Certificate Content-Type before writing headers. (#3304)
Set the Certificate Content-Type before writing headers. (#3304)

Prior to this commit we called `response.WriteHeader(http.StatusOK)` in
the wfe2's `Certificate` handler before
`response.Header().Set("Content-Type", "")`, which meant that the
response was served with a Content-Type of `application/json` instead of
the intended `application/pem-certificate-chain`. This commit changes
the order so the correct Content-Type header is written in the response.
jcjones/boulder,bmw/boulder,jfrazelle/boulder,lucasallan/boulder,jfrazelle/boulder,bmw/boulder,jcjones/boulder,jcjones/boulder,jsha/boulder,letsencrypt/boulder,lucasallan/boulder,bmw/boulder,jfrazelle/boulder,lucasallan/boulder,jsha/boulder,jfrazelle/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,letsencrypt/boulder