Fix #2317 force conversion to FormData for xhr.send (#2481)

Couldn't get the proxy object to work properly with XMLHTTPRequest's send method.
Even though calls are being forwarded to its underlying FormData through the proxy, the request won't set its type to multipart/form-data when appropriate and instead send a text/plain request

src/htmx.js

@@ -3600,7 +3600,9 @@ var htmx = (function() {
       return encodedParameters
     } else {
       if (usesFormData(elt)) {
-        return formDataFromObject(filteredParameters)
+        // Force conversion to an actual FormData object in case filteredParameters is a formDataProxy
+        // See https://github.com/bigskysoftware/htmx/issues/2317
+        return overrideFormData(new FormData(), formDataFromObject(filteredParameters))
       } else {
         return urlEncode(filteredParameters)
       }