Enable -project option also in switching to existing vimfiler buffer

I can open project home (in which `.git` directory exists) with `:VimFiler -project` in first time,
but I cannot in second time.
It seems that `-project` option does not work when `:VimFiler` opens existing vimfiler buffer.
I found `vimfiler#init#_switch_vimfiler` does not check `context.project` and fixed it.