LSIF: fix JSON header in response

Created by: chrismwendt

I noticed some of these errors from lsif-server on my test cluster:

cannot set headers after they are sent to the client

It turns out I was using .json() incorrectly https://expressjs.com/en/api.html#res.json

There are no other uses of .json() in the file, nor .header().

Test plan: I deployed to a test cluster to verify it works