Double-encoding of ampersands in WYSIWYG image alt and title text

If you insert an image which has a character in its alt or title text that is encoded as an entity, such as an apostrophe, every time you save the page, the ampersand part of the entity code gets encoded again.

Sample after 4 saves:

<p>This page tests encoding of HTML entities in image alt and title text.</p>
<p>For comparison, here are some textual things that should be encoded as entities...</p>
<ol>
<li>Apostrophes/Single quotes: 'Quoted text'</li>
<li>Double quotes: "Quoted text"</li>
<li>Angle brackets: 3.2 &lt; 54 and x &gt; y.</li>
<li>Ampersands: Salt &amp; pepper. Rice &amp; beans.</li>
</ol>
<p>Image with alt and title text that contains apostrophes:</p>
<p><img alt="Green grass and blue sky&amp;amp;amp;amp;amp;#039;s the limit" title="Cool pic of grass from the ant&amp;amp;amp;amp;amp;#039;s eye view" class="panopoly-image-quarter media-element file-teaser panopoly-image-quarter" src="/sites/default/files/styles/panopoly_image_quarter/public/7787881458_ebda60dbaa_b.jpg?itok=23Xan-2g" /></p>
<p>Saved once.<br />Saved twice.<br />Saved three times.<br />Saved 4 times.</p>