Use count(:all) for associations in HasMany field (thoughtbot#721)

Problem: Calling `count` on an association can cause invalid SQL queries
to be created where the `SELECT COUNT(a, b, c)` function receives
multiple  columns. This will cause a `StatementInvalid` exception later
on.

Solution: Use `count(:all)`, which generates a `SELECT COUNT(*)...`
query independently of the association.

app/views/fields/has_many/_show.html.erb

@@ -30,7 +30,7 @@ from the associated resource class's dashboard.
       <%= t(
         'administrate.fields.has_many.more',
         count: field.limit,
-        total_count: field.data.count,
+        total_count: field.data.count(:all),
       ) %>
     </span>
   <% end %>

lib/administrate/field/has_many.rb

@@ -41,7 +41,7 @@ def resources
       end
 
       def more_than_limit?
-        data.count > limit
+        data.count(:all) > limit
       end
 
       private

spec/support/mock_relation.rb

@@ -3,9 +3,14 @@ def initialize(data)
     @data = data
   end
 
-  delegate :==, :count, to: :@data
+  delegate :==, to: :@data
 
   def limit(n)
     @data.first(n)
   end
+
+  def count(column = nil)
+    return @data.count if column == :all
+    @data.count(column)
+  end
 end