If you want to specify UTF8 charset declaration in your module's response, you can define an After interceptor. Here's how you can define an After Interceptor in your module's constructor:
After += ctx =>
{
...
}
You can also define Application-level hook in your bootstrapper:
pipelines.AfterRequest += (ctx) => { ... };
Update: Based on the comments, best approach would be using the following code in the hook:
if (ctx.Response.ContentType == "text/html")
ctx.Response.ContentType = "text/html; charset=utf-8";