I took a freshly-cloned copy of the Boomerang library and ran make to produce the default boomerang.js script.
I copied that file, along with the following HTML document into a directory.
<html>
<head>
<script src="/boomerang.js" type="text/javascript"></script>
<script type="text/javascript">
BOOMR.init({
user_ip: "127.12.34.56",
beacon_url: "http://127.0.0.1:8008/beacon",
beacon_type: "POST"
});
</script>
</head>
<body>
<h1>Hello RUM</h1>
</body>
</html>
I then served that out of host running on port 8008 and viewed the html file. When I check the access log, I see
127.0.0.1 - - [17/May/2016:15:13:31 +0200] "GET /beacon?u=http%3A%2%2F127.0.0.1%3A8008%2Frum.html&v=0.9&...
That is, the Boomerang library is issuing a GET instead of POST and at a quick glance I can't figure out why. (The /beacon endpoint doesn't actually exist, so it throws a 404, but that is (I hope) beside the point). I'll take a closer look with dev tools, but has anyone experienced this before?