Reuse body encoding

This prevents emails from being garbled when delivered.

Closes #166

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## HEAD
 
 - Improve check for Rails module
+- Preserve body encoding to prevent garbled mails
 
 ## v1.9.3
 

lib/premailer/rails/hook.rb

@@ -70,17 +70,27 @@ def generate_html_part
         # can end up containing CSS rules.
         generate_text_part if generate_text_part?
 
-        Mail::Part.new(
-          content_transfer_encoding: html_part.content_transfer_encoding,
-          content_type: "text/html; charset=#{html_part.charset}",
-          body: premailer.to_inline_css)
+        part = html_part
+        html = premailer.to_inline_css
+        Mail::Part.new do
+          content_transfer_encoding part.content_transfer_encoding
+          content_type "text/html; charset=#{part.charset}"
+          body html
+          body_encoding part.body.encoding
+        end
       end
 
       def generate_text_part
-        @text_part ||= Mail::Part.new(
-          content_transfer_encoding: html_part.content_transfer_encoding,
-          content_type: "text/plain; charset=#{html_part.charset}",
-          body: premailer.to_plain_text)
+        @text_part ||= begin
+          part = html_part
+          text = premailer.to_plain_text
+          Mail::Part.new do
+            content_transfer_encoding part.content_transfer_encoding
+            content_type "text/plain; charset=#{part.charset}"
+            body text
+            body_encoding part.body.encoding
+          end
+        end
       end
 
       def premailer

spec/integration/hook_spec.rb

@@ -5,6 +5,10 @@ def run_hook(message)
     Premailer::Rails::Hook.perform(message)
   end
 
+  def body_content(message)
+    Nokogiri::HTML(message.html_string).at('body').content
+  end
+
   class Mail::Message
     def html_string
       (html_part || self).body.to_s
@@ -54,6 +58,18 @@ def html_string
     end
   end
 
+  it 'does not screw up the text by maintaining the original body encoding' do
+    raw_msg = Fixtures::Message.latin_message
+    processed_msg = Fixtures::Message.latin_message
+    run_hook(processed_msg)
+    expect(body_content(processed_msg)).to eq(body_content(raw_msg))
+
+    raw_msg = Fixtures::Message.non_latin_message
+    processed_msg = Fixtures::Message.non_latin_message
+    run_hook(processed_msg)
+    expect(body_content(processed_msg)).to eq(body_content(raw_msg))
+  end
+
   it 'generates a text part from the html' do
     expect { run_hook(message) }.to change(message, :text_part)
   end

spec/support/fixtures/message.rb

@@ -114,6 +114,22 @@ def with_body(body_type)
       message
     end
 
+    def latin_message
+      base_message.tap do |message|
+        message.body = HTML_PART
+        message.content_type 'text/html; charset=UTF-8'
+        message.ready_to_send!
+      end
+    end
+
+    def non_latin_message
+      base_message.tap do |message|
+        message.body = HTML_PART_WITH_UNICODE
+        message.content_type 'text/html; charset=UTF-8'
+        message.ready_to_send!
+      end
+    end
+
     private
 
     def base_message