Fix aggregate for combination queries

[iguberman]

Fixes #99)

[drewolson]

@iguberman thanks for the PR! I've noticed you've been incrementally adding some commits. Is this ready for review / merge or are you still working on it?

[kelostrada]

==> scrivener_ecto
Compiling 2 files (.ex)
warning: variable "caller" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/scrivener/paginater/ecto/query.ex:48: Scrivener.Paginater.Ecto.Query.total_entries/4

warning: variable "count_query" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/scrivener/paginater/ecto/query.ex:57: Scrivener.Paginater.Ecto.Query.total_entries/4

warning: variable "prefix" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/scrivener/paginater/ecto/query.ex:49: Scrivener.Paginater.Ecto.Query.total_entries/4

warning: variable "expr" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/scrivener/paginater/ecto/query.ex:89: Scrivener.Paginater.Ecto.Query.aggregate/1

please fix the warnings.

Also run: mix format