Fix another incompatible character encoding error

In fae72e5 we introduced a fallback for incompatible character
encodings, but that only covered the case where Mapi::Mime#is_multipart?
returns true.

This is a slight variation for the other branch of the conditional.

The error is being raised where we have a `@body` in a different
encoding to the default string encoding. In this case, just always force
the string to be the same encoding as `@body` so we don’t get an error.

This passes the spec for the particular file we’re seeing problems with,
but I’m not at all sure this is the right thing to do. Maybe we should
always treat anything in mapi as ASCII, and then force to UTF-8 for
display? [1]

Fixes mysociety/alaveteli#5783

[1] aquasync#9 (comment)

Author: garethrees

lib/mapi/mime.rb

@@ -126,7 +126,7 @@ def to_s opts={}
   		@headers.each do |key, vals|
   			vals.each { |val| str << "#{key}: #{val}\r\n" }
   		end
-  		str << "\r\n" + @body
+  		str.force_encoding(@body.encoding) << "\r\n" + @body
   	end
 
   	def self.split_header header

test/28101-outlook-attachment.msg

@@ -44,6 +44,15 @@ def test_multipart_rendered_string_is_valid_encoding
     end
   end
 
+  def test_non_multipart_rendered_string_is_valid_encoding
+    msg = Mapi::Msg.open "#{TEST_DIR}/28101-outlook-attachment.msg" do |msg|
+      string_version = msg.to_mime.to_s
+      if string_version.respond_to?(:valid_encoding?)
+        assert_equal true, string_version.valid_encoding?
+      end
+    end
+  end
+
   def test_embedded_msg_renders_as_string
     msg = Mapi::Msg.open "#{TEST_DIR}/embedded.msg" do |msg|
       assert_match "message/rfc822", msg.to_mime.to_s