INT-3120: remove `Accept-Charset` from Response

`StringHttpMessageConverter` adds `Accept-Charset` header by default.
By RFC 2616 `Accept-Charset` is a Request header,
so it won't be presented in the Response.

Turn off `writeAcceptCharset` in the `HttpRequestHandlingEndpointSupport`.

JIRA: https://jira.springsource.org/browse/INT-3120