Fix total entries when query has combinations

[oo6]

Currently, Repo.paginate may throw an exception if the query has combinations.

Test case:

Post
|> Post.published()
|> union(^Post.unpublished(Post))
|> Scrivener.Ecto.Repo.paginate()

Exception message:

** (Postgrex.Error) ERROR 42601 (syntax_error) each UNION query must have the same number of columns
    query: SELECT count('*') FROM "posts" AS p0 WHERE (p0."published" = TRUE) UNION (SELECT p0."id", p0."title", p0."body", p0."published", p0."inserted_at", p0."updated_at" FROM "posts" AS p0 WHERE (p0."published" = FALSE))

Inspiration from ecto, I direct use Ecto.Query.aggregate to count total entries except query has group_bys.

[oo6]

@drewolson Hi! Do you have time to review this PR? I look forward to finishing it, thanks.

[cpjolicoeur]

@oo6 I know this is several years later, but we have just taken over maintenance of this repository. If you still are using scrivener and would like to get this PR included, can you please update off the latest master branch. Otherwise, we'll close this PR shortly if no activity.

[oo6]

@cpjolicoeur Hey, I've updated the PR. Thank you for taking over maintenance.