Set a 'generic' filetype for not supported filetypes

Since `ycmd-mode` is used to check whether completion should work, we have to add some filetype in the request. The filetype cannot be nil or an empty string in the request. 
`generic` is fine. You just need to use _something_. Just be consistent about it for files that don't have a filetype; like @ptrv said, identifiers will be grouped under that name.