Fix compatibility issue with Mail 2.7.0

In Mail 2.7.0 in an actual Rails application, HTML part of the body now
returns an `ActiveSupport::SafeBuffer` object instead of a String
object. This causes a problem as there is a known issue of how
`SafeBuffer#gsub` with a block form is broken in Rails[1], and that
`HTMLEntities#decode` actually performs a `gsub` with a block
internally[2].

Upon further investigation, however, it seems like the root cause of
this issue might not be on Mail gem, but actually on `ERB::Util.h`
returning `ActiveSupport::SafeBuffer` object instead of `String`[3].

This commit changes `default_part_body` method to call `to_s.to_str` on
the message body so that we'll be able to pass a String object, which
always works with `gsub`, to `HTMLEntities#decode`.

Please note that we need to call `to_s.to_str` on the object because
`ActiveSupport::SafeBuffer` actually overrides `to_s` to return itself
and not the underlying `String` object.

I believe this PR should fix issue #202, #204, and #205, and it's better
to fix the issue here than in `HTMLEntities`.

[1]: rails/rails#1555
[2]: https://github.com/threedaymonk/htmlentities/blob/v4.3.3/lib/htmlentities/decoder.rb#L10-L20
[3]: threedaymonk/htmlentities#33 (comment)

lib/email_spec/mail_ext.rb

@@ -4,7 +4,8 @@ def default_part
   end
 
   def default_part_body
-    HTMLEntities.new.decode(default_part.body)
+    # Calling to_str as we want the actual String object
+    HTMLEntities.new.decode(default_part.body.to_s.to_str)
   end
 
   def html

spec/email_spec/mail_ext_spec.rb

@@ -30,6 +30,11 @@
       email = Mail.new(:body => "hi")
       expect(email.default_part.body).to eq(email.default_part_body)
     end
+
+    it "compatible with ActiveSupport::SafeBuffer" do
+      email = Mail.new(:body => ActiveSupport::SafeBuffer.new("bacon & pancake"))
+      expect(email.default_part_body).to eq ("bacon & pancake")
+    end
   end
 
   describe "#html" do