Handle incomptible encodings in email headers

lib/mail/encodings.rb

@@ -134,7 +134,15 @@ def Encodings.value_decode(str)
           when *Q_VALUES then q_value_decode(string)
           end
         end
-      end.join("")
+      end
+
+      if lines.each_slice(2).all? { |x, y| Encoding.compatible?(x, y) }
+        lines.join("")
+      else
+        lines.map do |line|
+          line.encode("UTF-8", invalid: :replace, undef: :replace, replace: "�")
+        end.join("")
+      end
     end
 
     # Takes an encoded string of the format =?<encoding>?[QB]?<string>?=

spec/mail/encoding_spec.rb

@@ -189,6 +189,13 @@
     expect(m.subject).to eq "Hello #{replace} World"
   end
 
+  it "should handle incompatible encodings by force encoding to UTF-8" do
+    m = Mail.new
+    subject = "\xEC =?utf-8?Q?=F0=9F=98=80?=".dup.force_encoding("ASCII-8BIT")
+    m['Subject'] = Mail::SubjectField.new(subject)
+    expect(m.subject).to eq "� 😀"
+  end
+
   it "should replace characters of unknown and invalid encoding" do
     m = Mail.new
     m['Subject'] = Mail::SubjectField.new("Hello=?UNKNOWN?B?4g==?=")