Invalid plural: `Unequal Dictionarys`

Hi!

It's a detail, but it's bugging me and I thought I'd submit a pull request... but it's going to be more than a small "fix a typo', since it's just a types dictionary to which we add an `s`:

printf("Unequal %ss\n %%s should be equal to \n %%s", get(s:type_names, type))

I see a few options:

1. We change the sentence so no plural is required (e.g. `Unequal Dictionary instances` or change the display to `Compared type: Dictionary`)
2. We create two arrays, one `s:type_names` and the other, `s:type_names_plural`
3. We replace the `get` call by a dedicated function, `s:get_type_name`, that would receive a `v:bool` `plural` and would make an exception for the plural of `v:dict`.

All are pretty simple, but I'd like to know what you want to do before investing time in this.

Another thing; `vader.vader` fails in both `Ubuntu 16.04` with `vim 7.4.1689`, and on Windows with `8.0.442`, are all tests supposed to be passing?

Finally, I didn't see any tests for `assert.vim`, I can add some.

What do you think?