removed unnecessary type conversion and now using append in chain.Append

chain.go

diff --git a/chain.go b/chain.go
index c75f9c9..58a7a51 100644
--- a/chain.go
+++ b/chain.go
@@ -66,7 +66,7 @@
 	if fn == nil {
 		return c.Then(nil)
 	}
-	return c.Then(http.HandlerFunc(fn))
+	return c.Then(fn)
 }
 
 // Append extends a chain, adding the specified constructors
@@ -79,9 +79,9 @@
 //     // requests in stdChain go m1 -> m2
 //     // requests in extChain go m1 -> m2 -> m3 -> m4
 func (c Chain) Append(constructors ...Constructor) Chain {
-	newCons := make([]Constructor, len(c.constructors)+len(constructors))
-	copy(newCons, c.constructors)
-	copy(newCons[len(c.constructors):], constructors)
+	newCons := make([]Constructor, 0, len(c.constructors)+len(constructors))
+	newCons = append(newCons, c.constructors...)
+	newCons = append(newCons, constructors...)
 
 	return New(newCons...)
 }