修复SaServletFilter异常函数中无法自定义Content-Type的问题

sa-token-starter/sa-token-spring-boot-starter/src/main/java/cn/dev33/satoken/filter/SaServletFilter.java

@@ -167,7 +167,9 @@ public class SaServletFilter implements Filter {
 			String resultString = String.valueOf(result);
 			
 			// 2. 写入输出流 
-			response.setContentType("text/plain; charset=utf-8"); 
+			if(response.getContentType() == null) {
+				response.setContentType("text/plain; charset=utf-8"); 
+			}
 			response.getWriter().print(resultString);
 			return;
 		}