Use frozen_string_literal in all core filters

lib/docs/filters/core/attribution.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class AttributionFilter < Filter
     def call

lib/docs/filters/core/clean_html.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class CleanHtmlFilter < Filter
     def call

lib/docs/filters/core/clean_local_urls.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class CleanLocalUrlsFilter < Filter
     def call

lib/docs/filters/core/clean_text.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class CleanTextFilter < Filter
     EMPTY_NODES_RGX = /<(?!td|th|iframe|mspace)(\w+)[^>]*>[[:space:]]*<\/\1>/

lib/docs/filters/core/container.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class ContainerFilter < Filter
     class ContainerNotFound < StandardError; end

lib/docs/filters/core/entries.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class EntriesFilter < Filter
     def call

lib/docs/filters/core/inner_html.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class InnerHtmlFilter < Filter
     def call

lib/docs/filters/core/internal_urls.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class InternalUrlsFilter < Filter
     def call

lib/docs/filters/core/normalize_paths.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class NormalizePathsFilter < Filter
     def call

lib/docs/filters/core/normalize_urls.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class NormalizeUrlsFilter < Filter
     ATTRIBUTES = { a: 'href', img: 'src', iframe: 'src' }

lib/docs/filters/core/title.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docs
   class TitleFilter < Filter
     def call