patch 8.0.0533: abbreviation doesn't work after backspacing newline

Problem:    Abbreviation doesn't work after backspacing newline. (Hkonrk)
Solution:   Set the insert start column. (closes #1609)
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index fa8012a..ab02a87 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -160,3 +160,14 @@
   set nomodified
   iunmap <M-">
 endfunc
+
+func Test_abbr_after_line_join()
+  new
+  abbr foo bar
+  set backspace=indent,eol,start
+  exe "normal o\<BS>foo "
+  call assert_equal("bar ", getline(1))
+  bwipe!
+  unabbr foo
+  set backspace&
+endfunc