Demonstrate how to get swagger.json

[opticyclic]

Add a demo on how to get the swagger.json file as it is sometimes required to upload to external sites that are hosting your documentation.

e.g.

@RunWith(SpringRunner.class)
@SpringBootTest
public class GenerateSwagger {

    @Autowired
    WebApplicationContext context;

    @Test
    public void generateSwagger() throws Exception {
        MockMvc mockMvc = MockMvcBuilders.webAppContextSetup(context).build();
        mockMvc.perform(MockMvcRequestBuilders.get("/v2/api-docs").accept(MediaType.APPLICATION_JSON))
                .andDo((result) -> {
                    FileUtils.writeStringToFile(new File("swagger.json"), result.getResponse().getContentAsString());
                });

    }
}

Originally posted by @yissachar in springfox/springfox#1959 (comment)