FIX: HtmlToMarkdown didn't keep text from within <center> tag

It should ignore the `<center>` tag, but keep the text from within the element.

lib/html_to_markdown.rb

@@ -220,7 +220,7 @@ class HtmlToMarkdown
     "\n\n#{traverse(node)}\n\n"
   end
 
-  TRAVERSABLES ||= %w[aside font span thead tbody tfoot u]
+  TRAVERSABLES ||= %w[aside font span thead tbody tfoot u center]
   TRAVERSABLES.each { |tag| define_method("visit_#{tag}") { |node| traverse(node) } }
 
   def visit_tt(node)

spec/lib/html_to_markdown_spec.rb

@@ -413,6 +413,10 @@ RSpec.describe HtmlToMarkdown do
     expect(html_to_markdown("<u>Underline</u>")).to eq("Underline")
   end
 
+  it "swallows <center>" do
+    expect(html_to_markdown("<center>Centered</center>")).to eq("Centered")
+  end
+
   it "removes <script>" do
     expect(html_to_markdown("<script>var foo = 'bar'</script>")).to eq("")
   end