Issue #28: do not set default Content-Type for empty response

header.go

@@ -1051,7 +1051,12 @@ func (h *ResponseHeader) AppendBytes(dst []byte) []byte {
 	dst = appendHeaderLine(dst, strServer, server)
 	dst = appendHeaderLine(dst, strDate, serverDate.Load().([]byte))
 
-	dst = appendHeaderLine(dst, strContentType, h.ContentType())
+	// Append Content-Type only for non-zero responses
+	// or if it is explicitly set.
+	// See https://github.com/valyala/fasthttp/issues/28 .
+	if h.ContentLength() != 0 || len(h.contentType) > 0 {
+		dst = appendHeaderLine(dst, strContentType, h.ContentType())
+	}
 
 	if len(h.contentLengthBytes) > 0 {
 		dst = appendHeaderLine(dst, strContentLength, h.contentLengthBytes)

header_regression_test.go

@@ -3,9 +3,43 @@ package fasthttp
 import (
 	"bufio"
 	"bytes"
+	"fmt"
+	"strings"
 	"testing"
 )
 
+func TestIssue28ResponseWithoutBodyNoContentType(t *testing.T) {
+	var r Response
+
+	// Empty response without content-type
+	s := r.String()
+	if strings.Contains(s, "Content-Type") {
+		t.Fatalf("unexpected Content-Type found in response header with empty body: %q", s)
+	}
+
+	// Explicitly set content-type
+	r.Header.SetContentType("foo/bar")
+	s = r.String()
+	if !strings.Contains(s, "Content-Type: foo/bar\r\n") {
+		t.Fatalf("missing explicitly set content-type for empty response: %q", s)
+	}
+
+	// Non-empty response.
+	r.Reset()
+	r.SetBodyString("foobar")
+	s = r.String()
+	if !strings.Contains(s, fmt.Sprintf("Content-Type: %s\r\n", defaultContentType)) {
+		t.Fatalf("missing default content-type for non-empty response: %q", s)
+	}
+
+	// Non-empty response with custom content-type.
+	r.Header.SetContentType("aaa/bbb")
+	s = r.String()
+	if !strings.Contains(s, "Content-Type: aaa/bbb\r\n") {
+		t.Fatalf("missing custom content-type: %q", s)
+	}
+}
+
 func TestIssue6RequestHeaderSetContentType(t *testing.T) {
 	testIssue6RequestHeaderSetContentType(t, "GET")
 	testIssue6RequestHeaderSetContentType(t, "POST")