FormHelper: convert params#to_unsafe_h only if Rails 5, add tests.

This adresses an issue correctly brought up by @ryanswood in PR #644
concerning Rails 4.2 where params#to_unsafe_h still needs to be
converted from a Hash to a HWIA to respond to symbol hash keys.

I prefer to trying adding a conditional on Active Record version rather
than an extra HWIA conversion.

The tests are run for Rails 4 and 5, since ActionController::Parameters
was backported to 4.x.

Rails 3 raises if ActionController::Parameters is invoked, so the tests
are skipped in that case.

This commit is necessary because when the following is run:

    ActionController::Parameters.new(q: 1).to_unsafe_h.fetch(:q)

with Rails 5 it works, but not with Rails 4.2. The reason is that in
Rails 5, #to_unsafe_h calls
StrongParameters::convert_parameters_to_hashes which converts hashes to
HWIA, whereas in Rails 4.2 it just calls #to_hash without HWIA.

lib/ransack/helpers/form_helper.rb

@@ -121,8 +121,11 @@ module Ransack
         private
 
           def parameters_hash(params)
-            return params unless params.respond_to?(:to_unsafe_h)
-            params.to_unsafe_h
+            if ::ActiveRecord::VERSION::MAJOR == 5 && params.respond_to?(:to_unsafe_h)
+              params.to_unsafe_h
+            else
+              params
+            end
           end
 
           def extract_sort_fields_and_mutate_args!(args)

spec/ransack/helpers/form_helper_spec.rb

@@ -329,18 +329,14 @@ module Ransack
           it { should match /exist\=existing/ }
         end
 
-        context 'using real ActionController Parameter object',
-          if: ::ActiveRecord::VERSION::MAJOR > 4 do
-
-          describe '#sort_link should include search params' do
+        context 'using a real ActionController::Parameter object',
+          if: ::ActiveRecord::VERSION::MAJOR > 3 do
 
+          describe '#sort_link should include search params with symbol q:' do
             subject { @controller.view_context.sort_link(Person.search, :name) }
-
-            let(:params) {
-              ActionController::Parameters
-              .new({ 'q' => { name_eq: 'TEST' }, controller: 'people' })
-            }
-
+            let(:params) { ActionController::Parameters.new(
+              { :q => { name_eq: 'TEST' }, controller: 'people' }
+              ) }
             before { @controller.instance_variable_set(:@params, params) }
 
             it {
@@ -351,6 +347,21 @@ module Ransack
             }
           end
 
+          describe "#sort_link should include search params with string 'q'" do
+            subject { @controller.view_context.sort_link(Person.search, :name) }
+            let(:params) {
+              ActionController::Parameters.new(
+                { 'q' => { name_eq: 'Test2' }, controller: 'people' }
+                ) }
+            before { @controller.instance_variable_set(:@params, params) }
+
+            it {
+              should match(
+                /people\?q(%5B|\[)name_eq(%5D|\])=Test2&q(%5B|\[)s(%5D|\])
+                =name\+asc/x,
+              )
+            }
+          end
         end
       end