Serve easily dynamic files with DataFromReader context method (#1304)

* Add DataFromReader context method

* Replace fmt by strconv.FormatInt

* Add pull request link to README

render/reader.go

@@ -0,0 +1,36 @@
+package render
+
+import (
+	"io"
+	"net/http"
+	"strconv"
+)
+
+type Reader struct {
+	ContentType   string
+	ContentLength int64
+	Reader        io.Reader
+	Headers       map[string]string
+}
+
+// Render (Reader) writes data with custom ContentType and headers.
+func (r Reader) Render(w http.ResponseWriter) (err error) {
+	r.WriteContentType(w)
+	r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
+	r.writeHeaders(w, r.Headers)
+	_, err = io.Copy(w, r.Reader)
+	return
+}
+
+func (r Reader) WriteContentType(w http.ResponseWriter) {
+	writeContentType(w, []string{r.ContentType})
+}
+
+func (r Reader) writeHeaders(w http.ResponseWriter, headers map[string]string) {
+	header := w.Header()
+	for k, v := range headers {
+		if val := header[k]; len(val) == 0 {
+			header[k] = []string{v}
+		}
+	}
+}