Minor Description
In my opinion I believe the following changes should be made:
For the writeJSON method, the content type should be changed from "text/plain" to "application/json". It's not as 'friendly' to some browsers but is more technically correct. See http://www.ietf.org/rfc/rfc4627.txt
For the writeXML method, if you don't put the character encoding type into the same line at the content type, then the encoding type can be ignored and decoded as ASCII (see http://annevankesteren.nl/2005/03/text-xml). So you could change the content type from "text/xml" to "text/xml; charset=utf-8" but it's probably better to change it to "application/xml". See http://www.grauw.nl/blog/entry/489