test: test default content-type for render option

Part of tarantool/security#8

Author: ligurio

test/integration/http_server_requests_test.lua

@@ -365,3 +365,18 @@ g.test_request_object_methods = function()
     t.assert_equals(parsed_body.read_cached, 'hello mister',
         'non-json req:read_cached()')
 end
+
+g.test_content_type_header_with_render = function()
+    local httpd = g.httpd
+    httpd:route({
+        method = 'GET',
+        path = '/content_type',
+        file = 'helper.html.el'
+    }, function(tx)
+        return tx:render()
+    end)
+
+    local r = http_client.get(helpers.base_uri .. '/content_type')
+    t.assert_equals(r.status, 200)
+    t.assert_equals(r.headers['content-type'], 'text/html; charset=utf-8', 'content-type header')
+end