Fix aggregates

[bdtomlin]

Addresses issues with aggregate queries

See #292, #304, and #287

[paulcsmith]

A couple small comments. Overall looks awesome! :D

[paulcsmith]

I think this test would still be nice to have but maybe there is a reason for deleting it? Is it covered by another test?

[bdtomlin]

Yes, it's covered in a different test. I created a generic describe "#select_sum" do that handles the 2 non type specific tests here:

avram/spec/query_spec.cr

Lines 402 to 415 in 6925948

	describe "#select_sum" do
	it "works with chained where clauses" do
	UserBox.create &.total_score(2000)
	UserBox.create &.total_score(1000)
	UserBox.create &.total_score(3000)
	sum = UserQuery.new.total_score.gte(2000).total_score.select_sum
	sum.should eq 5000
	end
	it "returns nil if there are no records" do
	query_sum = UserQuery.new.age.select_sum
	query_sum.should be_nil
	end
	end

It seemed unnecessary to repeat for every type of integer and float. Let me know if you would like me to do something a little different.

[paulcsmith]

Ah I see! I think this was a good call 👍

[paulcsmith]

Nice catch. More robust to check the type 👍

[paulcsmith]

Looks awesome. Loving these new fixes and improved/more accurate type signatures in aggregates