Fix prefix match (#591)

dispatcher.go

@@ -49,12 +49,14 @@ func (c RespConditionFunc) HandleResp(resp *http.Response, ctx *ProxyCtx) bool {
 // requests to url 'http://host/x'
 func UrlHasPrefix(prefix string) ReqConditionFunc {
 	return func(req *http.Request, ctx *ProxyCtx) bool {
-		// Make sure to include the / as the first path character
+		// Make sure to include the / as the first path character when we do a match
+		// using the host
 		relativePath := req.URL.Path
 		if length := len(relativePath); length == 0 || (length > 0 && relativePath[0] != '/') {
 			relativePath = "/" + relativePath
 		}
-		return strings.HasPrefix(relativePath, prefix) ||
+		// We use the original value to distinguish between "" and "/" in the user specified string
+		return strings.HasPrefix(req.URL.Path, prefix) ||
 			strings.HasPrefix(req.URL.Host+relativePath, prefix) ||
 			// Scheme value is something like "https", we must include the :// characters
 			strings.HasPrefix(req.URL.Scheme+"://"+req.URL.Host+relativePath, prefix)