[Spring] Fix character conversion error

[ackintosh]

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Description of the PR

In order to run a Spring stub server which returns multibyte example value, we need set character encoding.

This PR fixes the error below:

2018-10-08 20:42:09.842 ERROR 10859 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException: java.io.CharConversionException: Not an ISO 8859-1 character: [こ]] with root cause

[ackintosh]

@bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)

[cbornet]

LGTM