continuous changes should have ndjson content-type

The response for _changes?feed=continious should probably have the content-type ndjson? http://ndjson.org/

Start by tracing the HTTP response for the _changes?feed=continious endpoint and compare its Content-Type with the NDJSON guidance at ndjson.org. No source file or test is named; done means the continuous-changes response advertises the appropriate ndjson content type and its behavior is covered by a test.

Written by the indexing model from the issue text.