add method for ProxyHttpServer object

[ttmars]

I will execute Proxy.OnResponse().DoFunc many times.

type ProxyHttpServer struct {
respHandlers []RespHandler
}

so，I hope clear respHandlers chain
please add method for ProxyHttpServer to clear respHandlers

add method like this:
func (proxy *ProxyHttpServer) ClearRespHandlers() {
proxy.respHandlers = proxy.respHandlers[0:0]
}

thank you

[ErikPelli]

What is the purpose of that change? Is there a case where this is useful?