Refactor Calculations#execute_grouped_calculation and clean AR test case

* When tried to use `Company#accounts` test/models/company.rb I got:

```
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column:
accounts.company_id: SELECT COUNT(*) AS count_all, "companies"."firm_id"
AS companies_firm_id FROM "companies" INNER JOIN "accounts" ON
"accounts"."company_id" = "companies"."id" GROUP BY "companies"."firm_id"
```

* The refactor on Calculations class was just to simplify the code

activerecord/lib/active_record/relation/calculations.rb

@@ -277,14 +277,8 @@ def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
       end
       group_fields = arel_columns(group_fields)
 
-      group_aliases = group_fields.map { |field|
-        column_alias_for(field)
-      }
-      group_columns = group_aliases.zip(group_fields).map { |aliaz,field|
-        [aliaz, field]
-      }
-
-      group = group_fields
+      group_aliases = group_fields.map { |field| column_alias_for(field) }
+      group_columns = group_aliases.zip(group_fields)
 
       if operation == 'count' && column_name == :all
         aggregate_alias = 'count_all'
@@ -300,7 +294,7 @@ def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
       ]
       select_values += select_values unless having_clause.empty?
 
-      select_values.concat group_fields.zip(group_aliases).map { |field,aliaz|
+      select_values.concat group_columns.map { |aliaz, field|
         if field.respond_to?(:as)
           field.as(aliaz)
         else
@@ -309,7 +303,7 @@ def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
       }
 
       relation = except(:group)
-      relation.group_values  = group
+      relation.group_values  = group_fields
       relation.select_values = select_values
 
       calculated_data = @klass.connection.select_all(relation, nil, relation.bound_attributes)

activerecord/test/models/company.rb

@@ -10,7 +10,6 @@ class Company < AbstractCompany
   has_one :dummy_account, :foreign_key => "firm_id", :class_name => "Account"
   has_many :contracts
   has_many :developers, :through => :contracts
-  has_many :accounts
 
   scope :of_first_firm, lambda {
     joins(:account => :firm).