Fix issue with using :join for continuations

In case 'formatoptions' contains "j" and the continued line starts with
a comment character, the "join" ends up removing the comment character.

We don't actually have to use `:join` here, the right `substitute`
should do it.

Author: AndrewRadev

autoload/sj/vim.vim

@@ -46,9 +46,8 @@ function! sj#vim#Join()
   endif
 
   if next_line =~ continuation_pattern.'\s'
-    " Then there's some whitespace after the \, rely on :join
-    keeppatterns exe next_lineno.'s/'.continuation_pattern.'//'
-    exe current_lineno.','.next_lineno.'join'
+    " Then there's some whitespace after the \, join with a space
+    keeppatterns exe current_lineno.'s/\n'.continuation_pattern.'\s*/ /'
   else
     " No whitespace, let's join them directly
     keeppatterns exe current_lineno.'s/\n'.continuation_pattern.'//'

spec/plugin/vim_spec.rb

@@ -67,4 +67,21 @@
       let foo = 2+2
     EOF
   end
+
+  # Fix for https://github.com/AndrewRadev/splitjoin.vim/issues/216
+  specify "backslashes interpreted as a comment" do
+    set_file_contents <<~EOF
+      let foo = foo(
+            \\ '!#;'.var,
+            \\ \#{curwin: 0})
+    EOF
+
+    vim.search('var')
+    join
+
+    assert_file_contents <<~EOF
+      let foo = foo(
+            \\ '!#;'.var, \#{curwin: 0})
+    EOF
+  end
 end