make query method used for select filters more flexible

As per my discussion with @t-anjan in activeadmin#1093

lib/active_admin/inputs/filter_select_input.rb

@@ -4,11 +4,11 @@ class FilterSelectInput < ::Formtastic::Inputs::SelectInput
       include FilterBase
 
       # When it's a HABTM or has_many association, Formtastic builds "object_ids".
-      # Metasearch requires "objects_id", hence the convoluted override.
-      #
-      # We use "_in" instead of "_eq" since it works for single or multiple values.
+      # That doesn't fit our scenario, so we override it here.
       def input_name
-        reflection ? "#{method}_id_in" : "#{method}_in"
+        name = method.to_s
+        name.concat '_id' if reflection
+        name.concat multiple? ? '_in' : '_eq'
       end
 
       # Include the "Any" option if it's a dropdown, but not if it's a multi-select.

spec/unit/filters/filter_form_builder_spec.rb

@@ -223,7 +223,7 @@ def filter(name, options = {})
       let(:body) { filter :author }
 
       it "should generate a select" do
-        body.should have_tag "select",             :attributes => { :name => "q[author_id_in]" }
+        body.should have_tag "select",             :attributes => { :name => "q[author_id_eq]" }
       end
       it "should set the default text to 'Any'" do
         body.should have_tag "option", "Any",      :attributes => { :value => "" }