Respect original `bra` and `ket` when changing function

[YuruiHong]

I personally prefer the vim-surround grammar. When using csff<function name> to change a function name for a language that does not use () for functions, e.g. Mathematica using [], my configuration is like:

augroup sandwich-ft-mma
  autocmd BufRead,BufNewFile *.wl let b:sandwich_magicchar_f_patterns = [
                              \   {
                              \     'header' : '\<\%(\h\k*\.\)*\h\k*',
                              \     'bra'    : '\[',
                              \     'ket'    : '\]',
                              \     'footer' : '',
                              \   },
                              \ ]
augroup END

For the function below:

f[x_] := x^2

When my cursor is in the brackets, dsf can successfully delete the function to make it x_. However, when I tried to rename f to g using csffg, the result was:

g(x_) := x^2

It seems the [] brackets matched by bra and ket patterns were ignored in the final output, and the behavior of csff always assumes () are used for function calls. I wonder if it's better to respect the original surrounding characters.