Save, set, and restore startofline

Consider this code:
```javascript
_.map(
  (
    mapObjOnK
  )
);
```
If the cursor is on the 3rd line and you press <kbd>d</kbd><kbd>s</kbd><kbd>b</kbd>, the code will become
```javascript
_.map(
);
mapObjOnK
```

The problem occurs if `startofline` is off.

With this change, I set it before processing the the call to
`dosurround`, and then, after the call, I'm unsetting it if
it was unset.

plugin/surround.vim

@@ -367,6 +367,8 @@ function! s:reindent() abort " {{{1
 endfunction " }}}1
 
 function! s:dosurround(...) " {{{1
+  let sol_save = &startofline
+  set startofline
   let scount = v:count1
   let char = (a:0 ? a:1 : s:inputtarget())
   let spc = ""
@@ -388,6 +390,9 @@ function! s:dosurround(...) " {{{1
   if a:0 > 1
     let newchar = a:2
     if newchar == "\<Esc>" || newchar == "\<C-C>" || newchar == ""
+      if !sol_save
+        set nostartofline
+      endif
       return s:beep()
     endif
   endif
@@ -414,6 +419,9 @@ function! s:dosurround(...) " {{{1
   if keeper == ""
     call setreg('"',original,otype)
     let &clipboard = cb_save
+    if !sol_save
+      set nostartofline
+    endif
     return ""
   endif
   let oldline = getline('.')
@@ -478,6 +486,9 @@ function! s:dosurround(...) " {{{1
   else
     silent! call repeat#set("\<Plug>C".(a:0 > 2 && a:3 ? "S" : "s")."urround".char.newchar.s:input,scount)
   endif
+  if !sol_save
+    set nostartofline
+  endif
 endfunction " }}}1
 
 function! s:changesurround(...) " {{{1