This is related to
WW-3796: Error at setting character encoding if the parameters have been already read.
For all non-ajax requests, the default encoding is "UTF-8", defined in struts.properties: struts.i18n.encoding
For all ajax requests, the encoding is forced to "utf-8", defined in Dispatcher.java, line 684.
Note that the first is upper-case, the other is lower-case.
In the applyEncoding method, the encoding is compared case sensitive, so UTF-8 and utf-8 are not equal.
The patch changes utf-8 to UTF-8. The reference in the source comment at that line: http://www.w3.org/TR/XMLHttpRequest/#the-send-method says: "Let encoding be UTF-8."
This way the applyEncoding method will function as one would expect.