fix(available_element_definitions): Do not mutate element_definitions

`Page#available_element_definitions` returns elements still available
for that page. Taking unique and limited elements into account.

`Page#element_definitions` always returns the collections of defined
elements, not matter of they have been used or not.

Due to referencing the same instance variable and in memory object
`available_element_definitions` was mutating the `element_definitions`
object.

app/models/alchemy/page/page_elements.rb

@@ -80,21 +80,21 @@ def duplicate_elements(elements, repository, page_version)
       #       type: Richtext
       #
       def available_element_definitions(only_element_named = nil)
-        @_element_definitions ||= if only_element_named
+        @_available_element_definitions ||= if only_element_named
           definition = Element.definition_by_name(only_element_named)
           element_definitions_by_name(definition["nestable_elements"])
         else
-          element_definitions
+          element_definitions.dup
         end
 
-        return [] if @_element_definitions.blank?
+        return [] if @_available_element_definitions.blank?
 
         existing_elements = draft_version.elements.not_nested
         @_existing_element_names = existing_elements.pluck(:name)
         delete_unique_element_definitions!
         delete_outnumbered_element_definitions!
 
-        @_element_definitions
+        @_available_element_definitions
       end
 
       # All names of elements that can actually be placed on current page.
@@ -186,18 +186,18 @@ def generate_elements
         end
       end
 
-      # Deletes unique and already present definitions from @_element_definitions.
+      # Deletes unique and already present definitions from @_available_element_definitions.
       #
       def delete_unique_element_definitions!
-        @_element_definitions.delete_if do |element|
+        @_available_element_definitions.delete_if do |element|
           element["unique"] && @_existing_element_names.include?(element["name"])
         end
       end
 
-      # Deletes limited and outnumbered definitions from @_element_definitions.
+      # Deletes limited and outnumbered definitions from @_available_element_definitions.
       #
       def delete_outnumbered_element_definitions!
-        @_element_definitions.delete_if do |element|
+        @_available_element_definitions.delete_if do |element|
           outnumbered = @_existing_element_names.select { |name| name == element["name"] }
           element["amount"] && outnumbered.count >= element["amount"].to_i
         end

spec/models/alchemy/page_spec.rb

@@ -590,6 +590,12 @@ module Alchemy
           expect(subject.collect { |e| e["name"] }).to include("article")
           expect(subject.collect { |e| e["name"] }).not_to include("header")
         end
+
+        it "does not mutate the element_definitions collection" do
+          expect(page.element_definitions.collect { |e| e["name"] }).to include("header")
+          subject
+          expect(page.element_definitions.collect { |e| e["name"] }).to include("header")
+        end
       end
 
       context "limited amount" do
@@ -650,6 +656,12 @@ module Alchemy
         it "should be ignored if unique" do
           expect(subject.collect { |e| e["name"] }).not_to include("unique_headline")
         end
+
+        it "does not mutate the element_definitions collection" do
+          expect(page.element_definitions.collect { |e| e["name"] }).to include("column_headline")
+          subject
+          expect(page.element_definitions.collect { |e| e["name"] }).to include("column_headline")
+        end
       end
 
       describe ".ransackable_scopes" do