patch 8.1.1729: heredoc with trim not properly handled in function

Problem:    Heredoc with trim not properly handled in function.
Solution:   Allow for missing indent. (FUJIWARA Takuya, closes #4713)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 3690082..3bb5748 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -188,6 +188,15 @@
   call delete('XheredocBadMarker')
 endfunc
 
+func Test_let_heredoc_trim_no_indent_marker()
+  let text =<< trim END
+  Text
+  with
+  indent
+END
+  call assert_equal(['Text', 'with', 'indent'], text)
+endfunc
+
 " Test for the setting a variable using the heredoc syntax
 func Test_let_heredoc()
   let var1 =<< END