Use placeholders instead of placeholder for I18n, following the conventions

lib/simple_form/inputs/string_input.rb

@@ -29,7 +29,7 @@ module SimpleForm
       end
 
       def placeholder
-        @placeholder ||= options[:placeholder] || translate(:placeholder)
+        @placeholder ||= options[:placeholder] || translate(:placeholders)
       end
     end
   end

test/form_builder_test.rb

@@ -231,7 +231,7 @@ class FormBuilderTest < ActionView::TestCase
   end
 
   test 'placeholder should not be generated when set to false' do
-    store_translations(:en, :simple_form => { :placeholder => { :user => {
+    store_translations(:en, :simple_form => { :placeholders => { :user => {
       :name => 'Name goes here'
     } } }) do
       with_form_for @user, :name, :placeholder => false

test/inputs_test.rb

@@ -59,7 +59,7 @@ class InputTest < ActionView::TestCase
   end
 
   test 'input should use i18n to translate placeholder text' do
-    store_translations(:en, :simple_form => { :placeholder => { :user => {
+    store_translations(:en, :simple_form => { :placeholders => { :user => {
       :name => 'Name goes here'
     } } }) do
       with_input_for @user, :name, :string