The /api/main/doc endpoint was returning HTML content with a
text/plain content-type header instead of text/html. This caused
browsers to render the response as plain text.
Added content-type: text/html; charset=utf-8 header to the
response in the doc handler and added a regression test to
verify the fix.
Closes#9680
Signed-off-by: Andrey Antukh <niwi@niwi.nz>
