Allow passing :with argument to :be_rendered

We need it in order to check the rendered content.

spec/components/shared/link_list_component_spec.rb

@@ -17,9 +17,8 @@ describe Shared::LinkListComponent do
     render_inline Shared::LinkListComponent.new(
       ["Home", "/"], ["Info", "/info"], class: "menu"
     )
-    list = page.find("body").native.inner_html
 
-    expect(list).to eq '<ul class="menu">' + "\n" +
+    expect(page).to be_rendered with: '<ul class="menu">' + "\n" +
       '<li><a href="/">Home</a></li>' + "\n" +
       '<li><a href="/info">Info</a></li>' + "\n</ul>\n"
   end
@@ -28,9 +27,8 @@ describe Shared::LinkListComponent do
     render_inline Shared::LinkListComponent.new(
       '<a href="/">Home</a>'.html_safe, ["Info", "/info"], class: "menu"
     )
-    list = page.find("body").native.inner_html
 
-    expect(list).to eq '<ul class="menu">' + "\n" +
+    expect(page).to be_rendered with: '<ul class="menu">' + "\n" +
       '<li><a href="/">Home</a></li>' + "\n" +
       '<li><a href="/info">Info</a></li>' + "\n</ul>\n"
   end

spec/support/matchers/be_rendered.rb

@@ -1,6 +1,18 @@
-RSpec::Matchers.define :be_rendered do
+RSpec::Matchers.define :be_rendered do |with: nil|
   match do |page|
-    !page.native.inner_html.empty?
+    if with.nil?
+      !page.native.inner_html.empty?
+    else
+      page.has_css?("body") && page.find("body").native.inner_html == with
+    end
+  end
+
+  failure_message do |page|
+    if page.has_css?("body")
+      "expected page to be rendered with #{with}, but was rendered with #{page.find("body").native.inner_html}"
+    else
+      "expected page to be rendered with #{with}, but was not rendered"
+    end
   end
 
   failure_message_when_negated do |page|