Four-byte Unicode characters are not properly encoded in email notifications

When adding a comment containing a Unicode character with a four-byte UTF-8 representation, the character is not correctly encoded in email notifications.

For example, my comment today on JRA-36135 included the Unicode Pile of Poo character (💩). The correct HTML encoding for this entity is:

💩

However, in the email I received (on OS X 10.9 Mail.app), the text was rendered incorrectly as two separate characters (see screenshot).

The email contained the following HTML content:

                                            <p style="margin: 10px 0 0 0">How else are you going to add a Unicode Pile of Poo character (&#55357;&#56489;, 0xF0 0x9F 0x92 0xA9) to the issue comments?</p>