Fix reply-by-mail for 8-bit transfer encodings

The mail class seems to handle mails sent with Content-Transfer-Encoding: 8bit
somewhat weirdly: It decodes them (to utf-8), changes the raw source to base64,
and does not modify the Content-Type:charset= header.

This leads to Discourse trying the message encoding (in my example ISO-8859-1)
first, and if that does not contain any unparseable characters, it uses that.
Sadly, in ISO-8859-1, every byte sequence is valid.

Fix this by always trying to decode as UTF-8 first. The probability of someone
using another encoding that cleanly (but wrongly) decodes as UTF-8 should be
fairly low.

spec/components/email/receiver_spec.rb

@@ -158,7 +158,9 @@ describe Email::Receiver do
 
       expect { process(:html_reply) }.to change { topic.posts.count }
       expect(topic.posts.last.raw).to eq("This is a **HTML** reply ;)")
+    end
 
+    it "handles different encodings correctly" do
       expect { process(:hebrew_reply) }.to change { topic.posts.count }
       expect(topic.posts.last.raw).to eq("שלום! מה שלומך היום?")
 
@@ -167,6 +169,10 @@ describe Email::Receiver do
 
       expect { process(:reply_with_weird_encoding) }.to change { topic.posts.count }
       expect(topic.posts.last.raw).to eq("This is a reply with a weird encoding.")
+
+      expect { process(:reply_with_8bit_encoding) }.to change { topic.posts.count }
+      expect(topic.posts.last.raw).to eq("hab vergessen kritische zeichen einzufügen:\näöüÄÖÜß")
+
     end
 
     it "prefers text over html" do