chore(rails 5): selectively reverting encoding

premailer-rails made changes after 1.9.4 that broke Planning Center's
usage within pco-communication. Encoding would flip-flop between
ASCII-8BIT and UTF-8, and when this was attempting to go over the wire
it could not be decoded by Planning Center People.

This commit acts as a minimal patch, that removes the parts we don't
need so emails continue working as expected in Planning Center
applications using pco-communication on Rails 5.

There is an integration test for this inside Planning Center Giving. I
tried to write a test that could be generic and live in this fork of the
gem, but ultimately the entire system had to be exercised by hitting
`people.pco.dev` to encounter the error.

My hope is that by consolidating this patch to one commit, we can stay
up-to-date with premailer-rails as the project evolves, with minimal
maintenance.

Original work to get this problem solved was by @zhubert. Thanks Zack!

Moving to ministrycentered/premailer-rails and basing off of 1.9.7 by
@danott

lib/premailer/rails/customized_premailer.rb

@@ -13,7 +13,7 @@ def initialize(html)
         doc = load_html(html)
         options = @options.merge(css_string: CSSHelper.css_for_doc(doc))
 
-        super(doc.to_s, options)
+        super(html, options)
       end
     end
   end

lib/premailer/rails/hook.rb

@@ -73,10 +73,8 @@ def generate_html_part
         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
 
@@ -85,10 +83,8 @@ def generate_text_part
           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

spec/integration/hook_spec.rb

@@ -51,6 +51,7 @@ def html_string
     before { message.content_transfer_encoding = 'quoted-printable' }
 
     it 'should maintain the value' do
+      skip "Planning Center fork opts out of this for compatability with pco-communication"
       expect(processed_message.parts.first.content_transfer_encoding).to \
         eq 'quoted-printable'
       expect(processed_message.parts.last.content_transfer_encoding).to \