`MockHttpServletResponse` defaults to ISO-8859-1 for JSON while the rest of the framework defaults to UTF-8

[lukas-krecan]

Affects: 6.1.8

When Spring MVC serializes JSON and no charater encoding is specified, UTF-8 is used by default (see this or this).

MockMvc on the other hand always defaults to ISO-8859-1 which results in inconsistencies reproduced here.

This is a follow-up of #32997