Support default character encoding for response in MockMvc

[sbrannen]

Overview

#27214 introduced support for setting the default character encoding in MockHttpServletResponse, and the focus of this issue is to make that configurable when building an instance of MockMvc.

Deliverables

• Introduce defaultResponseCharacterEncoding(Charset) in ConfigurableMockMvcBuilder and use it to configure the default character encoding in the underlying MockHttpServletResponse.