Add Content-type header to Faraday request

[Andersond11]

Fixes an issue that was introduced when we moved to Rack 2.0.4 where a POST request without a content-type caused Rack to attempt to url decode the body. When this happened to a body that included % in any of it's contents, the request would blow up with ArgumentError - invalid %-encoding. The following two curl commands demonstrate the issue and solution locally:

This command without the content header will blow up with the above error:

curl -d'{"order":{"recipients":[{"line_items":[{"item_name":"NA","product_options":{"select-quantity":"%"}}]}]}}' http://localhost:5000/api/stores/9a5c1fafa3e1d7d45a9a3775a17e2622/orders?api_key=f9a7c8ebdfd34beaf260d9b0296c7059&api_timestamp=1525899406&api_signature=eeafc156e1ecf6fc02f49d5f9c17ce5f916f4a90390c072376943e5df05904a0

Adding the Content-type: application/json header results in the appropriate order validation errors, since the payload doesn't contain complete order data:

curl -H "Content-Type: application/json" -d'{"order":{"recipients":[{"line_items":[{"item_name":"NA","product_options":{"select-quantity":"%"}}]}]}}' http://localhost:5000/api/stores/9a5c1fafa3e1d7d45a9a3775a17e2622/orders?api_key=f9a7c8ebdfd34beaf260d9b0296c7059&api_timestamp=1525899406&api_signature=eeafc156e1ecf6fc02f49d5f9c17ce5f916f4a90390c072376943e5df05904a0

[brasic]

nice sleuthing!

[brasic]

Oops, think it's in the wrong place. shouldn't it be in post? GETs should not have bodies.

[brasic]

I think I recall that our GETs do sometimes have bodies, which is odd, but at any rate, the header should also be added to def post.

[Andersond11]

Yes, I fixed that in the bundled copy then put it in the wrong spot when I cloned the repo for the PR