Add Rails 7.1 Support (#1)

Adds and addresses the following:

* Adds Rails 7.1 to test matrix
* Addresses expected String type when rendering (zombocom#71)

gemfiles/rails_7_1.gemfile

@@ -0,0 +1,10 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "appraisal"
+gem "rails", "7.1.2"
+gem "sqlite3", "~> 1.4"
+gem 'pry', '~> 0.14.2'
+
+gemspec path: "../"

gemfiles/rails_7_1.gemfile.lock

@@ -0,0 +1,226 @@
+PATH
+  remote: ..
+  specs:
+    maildown (4.0.0)
+      actionmailer (>= 7.0.0)
+      kramdown-parser-gfm
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    actioncable (7.1.2)
+      actionpack (= 7.1.2)
+      activesupport (= 7.1.2)
+      nio4r (~> 2.0)
+      websocket-driver (>= 0.6.1)
+      zeitwerk (~> 2.6)
+    actionmailbox (7.1.2)
+      actionpack (= 7.1.2)
+      activejob (= 7.1.2)
+      activerecord (= 7.1.2)
+      activestorage (= 7.1.2)
+      activesupport (= 7.1.2)
+      mail (>= 2.7.1)
+      net-imap
+      net-pop
+      net-smtp
+    actionmailer (7.1.2)
+      actionpack (= 7.1.2)
+      actionview (= 7.1.2)
+      activejob (= 7.1.2)
+      activesupport (= 7.1.2)
+      mail (~> 2.5, >= 2.5.4)
+      net-imap
+      net-pop
+      net-smtp
+      rails-dom-testing (~> 2.2)
+    actionpack (7.1.2)
+      actionview (= 7.1.2)
+      activesupport (= 7.1.2)
+      nokogiri (>= 1.8.5)
+      racc
+      rack (>= 2.2.4)
+      rack-session (>= 1.0.1)
+      rack-test (>= 0.6.3)
+      rails-dom-testing (~> 2.2)
+      rails-html-sanitizer (~> 1.6)
+    actiontext (7.1.2)
+      actionpack (= 7.1.2)
+      activerecord (= 7.1.2)
+      activestorage (= 7.1.2)
+      activesupport (= 7.1.2)
+      globalid (>= 0.6.0)
+      nokogiri (>= 1.8.5)
+    actionview (7.1.2)
+      activesupport (= 7.1.2)
+      builder (~> 3.1)
+      erubi (~> 1.11)
+      rails-dom-testing (~> 2.2)
+      rails-html-sanitizer (~> 1.6)
+    activejob (7.1.2)
+      activesupport (= 7.1.2)
+      globalid (>= 0.3.6)
+    activemodel (7.1.2)
+      activesupport (= 7.1.2)
+    activerecord (7.1.2)
+      activemodel (= 7.1.2)
+      activesupport (= 7.1.2)
+      timeout (>= 0.4.0)
+    activestorage (7.1.2)
+      actionpack (= 7.1.2)
+      activejob (= 7.1.2)
+      activerecord (= 7.1.2)
+      activesupport (= 7.1.2)
+      marcel (~> 1.0)
+    activesupport (7.1.2)
+      base64
+      bigdecimal
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      connection_pool (>= 2.2.5)
+      drb
+      i18n (>= 1.6, < 2)
+      minitest (>= 5.1)
+      mutex_m
+      tzinfo (~> 2.0)
+    appraisal (2.5.0)
+      bundler
+      rake
+      thor (>= 0.14.0)
+    base64 (0.2.0)
+    bigdecimal (3.1.5)
+    builder (3.2.4)
+    coderay (1.1.3)
+    concurrent-ruby (1.1.10)
+    connection_pool (2.4.1)
+    crass (1.0.6)
+    digest (3.1.0)
+    drb (2.2.0)
+      ruby2_keywords
+    erubi (1.12.0)
+    globalid (1.0.0)
+      activesupport (>= 5.0)
+    i18n (1.10.0)
+      concurrent-ruby (~> 1.0)
+    io-console (0.7.1)
+    irb (1.11.0)
+      rdoc
+      reline (>= 0.3.8)
+    kramdown (2.4.0)
+      rexml
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
+    loofah (2.22.0)
+      crass (~> 1.0.2)
+      nokogiri (>= 1.12.0)
+    m (1.6.0)
+      method_source (>= 0.6.7)
+      rake (>= 0.9.2.2)
+    mail (2.7.1)
+      mini_mime (>= 0.1.1)
+    marcel (1.0.2)
+    method_source (1.0.0)
+    mini_mime (1.1.2)
+    minitest (5.15.0)
+    mutex_m (0.2.0)
+    net-imap (0.2.3)
+      digest
+      net-protocol
+      strscan
+    net-pop (0.1.1)
+      digest
+      net-protocol
+      timeout
+    net-protocol (0.1.3)
+      timeout
+    net-smtp (0.3.1)
+      digest
+      net-protocol
+      timeout
+    nio4r (2.7.0)
+    nokogiri (1.16.0-arm64-darwin)
+      racc (~> 1.4)
+    nokogiri (1.16.0-x86_64-darwin)
+      racc (~> 1.4)
+    nokogiri (1.16.0-x86_64-linux)
+      racc (~> 1.4)
+    pry (0.14.2)
+      coderay (~> 1.1)
+      method_source (~> 1.0)
+    psych (5.1.2)
+      stringio
+    racc (1.6.0)
+    rack (2.2.8)
+    rack-session (1.0.2)
+      rack (< 3)
+    rack-test (1.1.0)
+      rack (>= 1.0, < 3)
+    rackup (1.0.0)
+      rack (< 3)
+      webrick
+    rails (7.1.2)
+      actioncable (= 7.1.2)
+      actionmailbox (= 7.1.2)
+      actionmailer (= 7.1.2)
+      actionpack (= 7.1.2)
+      actiontext (= 7.1.2)
+      actionview (= 7.1.2)
+      activejob (= 7.1.2)
+      activemodel (= 7.1.2)
+      activerecord (= 7.1.2)
+      activestorage (= 7.1.2)
+      activesupport (= 7.1.2)
+      bundler (>= 1.15.0)
+      railties (= 7.1.2)
+    rails-dom-testing (2.2.0)
+      activesupport (>= 5.0.0)
+      minitest
+      nokogiri (>= 1.6)
+    rails-html-sanitizer (1.6.0)
+      loofah (~> 2.21)
+      nokogiri (~> 1.14)
+    railties (7.1.2)
+      actionpack (= 7.1.2)
+      activesupport (= 7.1.2)
+      irb
+      rackup (>= 1.0.0)
+      rake (>= 12.2)
+      thor (~> 1.0, >= 1.2.2)
+      zeitwerk (~> 2.6)
+    rake (13.0.6)
+    rdoc (6.6.2)
+      psych (>= 4.0.0)
+    reline (0.4.1)
+      io-console (~> 0.5)
+    rexml (3.2.5)
+    ruby2_keywords (0.0.5)
+    sqlite3 (1.4.2)
+    stringio (3.1.0)
+    strscan (3.0.3)
+    thor (1.3.0)
+    timeout (0.4.1)
+    tzinfo (2.0.4)
+      concurrent-ruby (~> 1.0)
+    webrick (1.8.1)
+    websocket-driver (0.7.6)
+      websocket-extensions (>= 0.1.0)
+    websocket-extensions (0.1.5)
+    zeitwerk (2.6.12)
+
+PLATFORMS
+  arm64-darwin-23
+  x86_64-darwin-20
+  x86_64-darwin-21
+  x86_64-linux
+
+DEPENDENCIES
+  appraisal
+  m
+  maildown!
+  pry (~> 0.14.2)
+  rails (= 7.1.2)
+  railties
+  rake
+  sqlite3 (~> 1.4)
+
+BUNDLED WITH
+   2.3.26

lib/maildown/markdown_engine.rb

@@ -20,10 +20,16 @@ module MarkdownEngine
     @maildown_markdown_engine_text_block = nil
 
     def self.to_html(string)
+      if string.is_a?(ActionView::OutputBuffer)
+        string = string.to_s
+      end
       html_block.call(string)
     end
 
     def self.to_text(string)
+      if string.is_a?(ActionView::OutputBuffer)
+        string = string.to_s
+      end
       text_block.call(string)
     end