Implement charset handling in WebRequestConcern

- The `force_encoding` option in WebsiteAgent is moved to
  WebRequestConcern so other users of the concern such as RssAgent can
  benefit from it.

- WebRequestConcern detects a charset specified in the Content-Type
  header to decode the content properly, and if it is missing the
  content is assumed to be encoded in UTF-8 unless it has a binary MIME
  type.  Not all Faraday adopters handle character encodings, and
  Faraday passes through what is returned from the backend, so we need
  to do this on our own. (cf. lostisland/faraday#139)

- WebRequestConcern now converts text contents to UTF-8, so agents can
  handle non-UTF-8 data without having to deal with encodings
  themselves.  Previously, WebsiteAgent in "json"/"text" modes and
  RssAgent would suffer from encoding errors when dealing with non-UTF-8
  contents.  WebsiteAgent in "html"/"xml" modes did not have this
  problem because Nokogiri would always return results in UTF-8
  independent of the input encoding.

This should fix #608.

app/concerns/web_request_concern.rb

@@ -14,6 +14,40 @@ def self.decode(val)
     end
   end
 
+  class CharacterEncoding < Faraday::Middleware
+    def initialize(app, force_encoding: nil, default_encoding: nil)
+      super(app)
+      @force_encoding   = force_encoding
+      @default_encoding = default_encoding
+    end
+
+    def call(env)
+      @app.call(env).on_complete do |env|
+        body = env[:body]
+
+        case
+        when @force_encoding
+          encoding = @force_encoding
+        when body.encoding == Encoding::ASCII_8BIT
+          # Not all Faraday adapters support automatic charset
+          # detection, so we do that.
+          case env[:response_headers][:content_type]
+          when /;(?:\s*,)?\s*charset\s*=\s*([^()<>@,;:\\\"\/\[\]?={}\s]+)/i
+            encoding = Encoding.find($1) rescue nil
+          when /\A\s*(?:text\/[^\s;]+|application\/[^\s;]+(?<=\b(?:xml|json))/i
+            encoding = @default_encoding
+          else
+            # Never try to transcode a binary content
+            return
+          end
+        end
+        body.encode!(Encoding::UTF_8, encoding) unless body.encoding == Encoding::UTF_8
+      end
+    end
+  end
+
+  Faraday::Response.register_middleware character_encoding: CharacterEncoding
+
   extend ActiveSupport::Concern
 
   def validate_web_request_options!
@@ -34,6 +68,23 @@ def validate_web_request_options!
     rescue ArgumentError => e
       errors.add(:base, e.message)
     end
+
+    if (encoding = options['force_encoding']).present?
+      case encoding
+      when String
+        begin
+          Encoding.find(encoding)
+        rescue ArgumentError
+          errors.add(:base, "Unknown encoding: #{encoding.inspect}")
+        end
+      else
+        errors.add(:base, "force_encoding must be a string")
+      end
+    end
+  end
+
+  def default_encoding
+    Encoding::UTF_8
   end
 
   def faraday
@@ -44,6 +95,10 @@ def faraday
     }
 
     @faraday ||= Faraday.new(faraday_options) { |builder|
+      builder.response :character_encoding,
+                       force_encoding: options['force_encoding'].presence,
+                       default_encoding: default_encoding
+
       builder.headers = headers if headers.length > 0
 
       builder.headers[:user_agent] = user_agent

app/models/agents/rss_agent.rb

@@ -29,6 +29,7 @@ class RssAgent < Agent
           * `basic_auth` - Specify HTTP basic auth parameters: `"username:password"`, or `["username", "password"]`.
           * `disable_ssl_verification` - Set to `true` to disable ssl verification.
           * `disable_url_encoding` - Set to `true` to disable url encoding.
+          * `force_encoding` - Set `force_encoding` to an encoding name if the website is known to respond with a missing, invalid or wrong charset in the Content-Type header.  Note that a text content without a charset is taken as encoded in UTF-8 (not ISO-8859-1).
           * `user_agent` - A custom User-Agent name (default: "Faraday v#{Faraday::VERSION}").
           * `max_events_per_run` - Limit number of events created (items parsed) per run for feed.
 

app/models/agents/website_agent.rb

@@ -87,7 +87,7 @@ class WebsiteAgent < Agent
 
       Set `uniqueness_look_back` to limit the number of events checked for uniqueness (typically for performance).  This defaults to the larger of #{UNIQUENESS_LOOK_BACK} or #{UNIQUENESS_FACTOR}x the number of detected received results.
 
-      Set `force_encoding` to an encoding name if the website does not return a Content-Type header with a proper charset.
+      Set `force_encoding` to an encoding name if the website is known to respond with a missing, invalid or wrong charset in the Content-Type header.  Note that a text content without a charset is taken as encoded in UTF-8 (not ISO-8859-1).
 
       Set `user_agent` to a custom User-Agent name if the website does not like the default value (`#{default_user_agent}`).