Fix writeBinaryFile to call the correct command

Fixes tauri-apps#1133

writeBinaryFile was reusing writeFile, which was happily saving
base64 encoded strings to the fs. This instead uses the correct
WriteBinaryFile command, which base64 decodes.

However why are we encoding and then decoding, why can we not just
send a raw byte array to be saved as a file? This is left for a later
PR.

.changes/writeBinaryFile.md

@@ -0,0 +1,13 @@
+---
+"tauri.js": patch
+"tauri": minor
+---
+
+fix writeBinaryFile in the js api to call the correct function
+on the rust side. Before, files were sent in base64 encoding, but
+the save-as-text `WriteFile` fs command was reused, without any 
+indication that the contents were base64 encoded. So the WriteFile
+command had no way of checking and decoding when necessary. 
+WriteBinaryFile on the rust side expects a base64 encoded string, then
+decodes it.
+

cli/tauri.js/api-src/fs.ts

@@ -166,7 +166,7 @@ async function writeBinaryFile(
   }
 
   return await promisified({
-    cmd: 'writeFile',
+    cmd: 'writeBinaryFile',
     path: file.path,
     contents: arrayBufferToBase64(file.contents),
     options