[SaaS] Missing charset on content-type header

In Tomcat, when you call response.setContentType("application/json") without explicitly setting the character encoding, the default character encoding of ISO-8859-1 is used. However, the default character encoding is not appended to the content type when you call response.getContentType().

So, if you haven't explicitly called response.setCharacterEncoding() to set a different character encoding, the returned value of response.getContentType() will be "application/json" without the character encoding appended.

However, if you explicitly invoke response.setCharacterEncoding("UTF-8"), then the character encoding "UTF-8" will be appended to the content type when you call response.getContentType(), resulting in "application/json;charset=UTF-8".

It turned out that in PageInfoRenderingValve#writeResponse we did never do something like

response.setCharacterEncoding(pageInfo.getCharacterEncoding());

before setting the content type on the response, and as a result, when the page would come from cache, it does not use the original charset but the default UTF-8 from the HST. With this fix we make sure that

1. We still have the default response encoding is UTF-8
2. If the HST rendering for some reason overrides the encoding, it also works for cached pages