doEndTag() is implemented like this:

public int doEndTag() throws JspException {
	if ((value == null)
	      && (pageContext.getRequest().getCharacterEncoding() == null)) {
	    /*
	     * no charset specified in tag or defined in request Content-Type
	     * header
	     */
	    value = (String) pageContext.findAttribute(REQUEST_CHAR_SET);
	}

	if (value != null) {
	    try {
		pageContext.getRequest().setCharacterEncoding(value);
	    } catch (UnsupportedEncodingException uee) {
		throw new JspTagException(uee.getMessage());
	    }
	}

	return EVAL_PAGE;
    }

If pageContext.getRequest().getCharacterEncoding() returns a non-null value, 
that value is ignored.