Allow AS::Cache::FileStore#clear without cache directory

Currently `Rails.cache.clear` raises Errno::ENOENT if it's run just
after cloning a new Rails project. It should succeed without removing
files or directories.

activesupport/lib/active_support/cache/file_store.rb

@@ -29,6 +29,7 @@ module ActiveSupport
       def clear(options = nil)
         root_dirs = Dir.entries(cache_path).reject {|f| (EXCLUDED_DIRS + [".gitkeep"]).include?(f)}
         FileUtils.rm_r(root_dirs.collect{|f| File.join(cache_path, f)})
+      rescue Errno::ENOENT
       end
 
       # Preemptively iterates through all stored keys and removes the ones which have expired.

activesupport/test/caching_test.rb

@@ -684,6 +684,7 @@ class FileStoreTest < ActiveSupport::TestCase
 
   def teardown
     FileUtils.rm_r(cache_dir)
+  rescue Errno::ENOENT
   end
 
   def cache_dir
@@ -703,6 +704,11 @@ class FileStoreTest < ActiveSupport::TestCase
     assert File.exist?(filepath)
   end
 
+  def test_clear_without_cache_dir
+    FileUtils.rm_r(cache_dir)
+    @cache.clear
+  end
+
   def test_long_keys
     @cache.write("a"*10000, 1)
     assert_equal 1, @cache.read("a"*10000)