Replace text parameter by format in Vuex API call.

Instead of { text: true } as a Fetch option, one can
now specify { format: "text" }, as well as any of these
other options, which call the corresponding Fetch method:

* "arrayBuffer",
* "blob",
* "formData",
* "json",
* "text"

lib/www/client/source/src/store/modules/api/actions.js

@@ -29,8 +29,16 @@ async function api ({state, getters, commit, dispatch}, [resource, init = {}, cb
 
       await dispatch('setCredentials');
 
+
       try {
-        return init.text ? (await res.text()) : (await res.json());
+        if (!res.bodyUsed) { // It may have been consumed by a callback
+          const validFormats = [ "arrayBuffer", "blob", "formData", "json", "text" ];
+          if (init.format && validFormats.includes(init.format)) {
+            return await res[init.format]();
+          } else {
+            return await res.json();
+          }
+        }
       } catch (err) {
         if (err instanceof SyntaxError) {
           if (Number(res.headers.get("Content-Length")) === 0) {