Update ref guide curl examples to eliminate Content-Type

SOLR-7217 added a special work arround to solr to autodetect the correct content-type if the user-agent indicates curl is in use and the content-type specified is the curl default ("application/x-www-form-urlencoded")

while this is certianly a nice to have convenience for people who forget to specify a Content-Type when using curl, there was quite a bit of back and forth on the ref guide TODO page about wether all example curl calls in the ref guide should explicitly take advantage of this for brevity.

I'm opening this issue in the interest of having a more structured place for this discussion.