Add a new have_html_escaped_body_text that match an HTML-escaped text

This solves transient failures when a text contains HTML-escapable
characters such as `'`.
Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/support/matchers/email_matchers.rb

+RSpec::Matchers.define :have_html_escaped_body_text do |expected|
+  match do |actual|
+    expect(actual).to have_body_text(ERB::Util.html_escape(expected))
+  end
+end