Sending DELETE request without id causing double slashes url path

[gaghan430]

so when we use something like collection.delete(null, { suffix: 'delete' }),
we expect to get /endpoint/delete, but what we get is /endpoint//delete

I guess we need to filter out falsy values from this segments https://github.com/mahdaen/metal-data/blob/master/src/request.ts#L84

[mahdaen]

@gaghan430 Good point to prevent falsy values, will work on that! But for such operation I suggest to use the .request() method instead, to keep the coding style clean.