fix: fx/fn.Head func will forever block when n is less than 1 (#128)

* fix fx/Stream Head func will forever block when n is less than 1

* update test case

* update test case

core/fx/fn.go

@@ -159,6 +159,9 @@ func (p Stream) Group(fn KeyFunc) Stream {
 }
 
 func (p Stream) Head(n int64) Stream {
+	if n < 1 {
+		panic("n must be greater than 0")
+	}
 	source := make(chan interface{})
 
 	go func() {