[RFC] vim-patch:7.4.455

```
Problem:  Completion for :buf does not use 'wildignorecase'. (Akshay H)
Solution: Pass the 'wildignorecase' flag around.

https://code.google.com/p/vim/source/detail?r=v7-4-455
```

This succeeds #1227. Adressed the comments in https://github.com/neovim/neovim/pull/1227/files#r18111197 and https://github.com/neovim/neovim/pull/1227/files#r17937514.