Fix encoding with 'utf-8-auto'

* src/coding.c (encode_coding_utf_8): Fix encoding with
'utf-8-auto': it should produce BOM, per the documentation of
the :bom attribute.  (Bug#60750)

* lisp/international/mule.el (define-coding-system): Doc fix.

* test/src/coding-tests.el (coding-tests): Use
'with-coding-priority' instead of 'prefer-coding-system', as the
latter has global persistent effect and affects further tests.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/coding.c b/src/coding.c
index 4e59f2b6a1b..49dcd8634f3 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -1431,7 +1431,7 @@ encode_coding_utf_8 (struct coding_system *coding)
   ptrdiff_t produced_chars = 0;
   int c;
 
-  if (CODING_UTF_8_BOM (coding) == utf_with_bom)
+  if (CODING_UTF_8_BOM (coding) != utf_without_bom)
     {
       ASSURE_DESTINATION (3);
       EMIT_THREE_BYTES (UTF_8_BOM_1, UTF_8_BOM_2, UTF_8_BOM_3);