util.dataforms: Convert media element sizes to avoid error on Lua 5.3

The stanza API does not accept number values and threw an error due to
the height and width attributes of the media element (XEP-0221).

This part had no test coverage previously, explaining why it was not
discovered until now.

spec/util_dataforms_spec.lua

@@ -106,6 +106,21 @@ describe("util.dataforms", function ()
 				name = "text-single-field",
 				value = "text-single-value",
 			},
+			{
+				-- XEP-0221
+				-- TODO Validate the XML produced by this.
+				type = "text-single",
+				label = "text-single-with-media-label",
+				name = "text-single-with-media-field",
+				media = {
+					height = 24,
+					width = 32,
+					{
+						type = "image/png",
+						uri = "data:",
+					},
+				},
+			},
 		});
 		xform = some_form:form();
 	end);