Fix streaming JSON Content-type for postBuild

Sorry for no test but the whole `postBuild()` does not seem to have any.
This also needs #8408 on the client side to properly work.


lineDelim is used by streamJSON() so it needs to be set
before its called.

Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com> (github: tonistiigi)

/cc @jfrazelle