Added more tests

Response.go

@@ -12,10 +12,10 @@ type Response interface {
 	io.Writer
 	io.StringWriter
 	Body() []byte
-	Header(key string) string
+	Header(string) string
 	SetHeader(key string, value string)
 	SetBody([]byte)
-	SetStatus(status int)
+	SetStatus(int)
 	Status() int
 }
 
@@ -44,9 +44,9 @@ func (res *response) Header(key string) string {
 
 // SetHeader sets the header value for the given key.
 func (res *response) SetHeader(key string, value string) {
-	for _, header := range res.headers {
+	for i, header := range res.headers {
 		if header.Key == key {
-			header.Value = value
+			res.headers[i].Value = value
 			return
 		}
 	}