net/http: return UnexpectedEOF instead of EOF on truncated resposne

Fixes #6564

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/52420043

src/pkg/net/http/response.go

@@ -141,6 +141,9 @@ func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
 	// Parse the response headers.
 	mimeHeader, err := tp.ReadMIMEHeader()
 	if err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
 		return nil, err
 	}
 	resp.Header = Header(mimeHeader)

src/pkg/net/http/response_test.go

@@ -618,6 +618,15 @@ func TestResponseContentLengthShortBody(t *testing.T) {
 	}
 }
 
+func TestReadResponseUnexpectedEOF(t *testing.T) {
+	br := bufio.NewReader(strings.NewReader("HTTP/1.1 301 Moved Permanently\r\n" +
+		"Location: http://example.com"))
+	_, err := ReadResponse(br, nil)
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("ReadResponse = %v; want io.ErrUnexpectedEOF", err)
+	}
+}
+
 func TestNeedsSniff(t *testing.T) {
 	// needsSniff returns true with an empty response.
 	r := &response{}