Enforce bpg action fields argument type is an object/dict

Summary:
If you pass a string, no error is throw, and no action is taken, causing a bit of confusion for users (such as cts)

refs T21563

Test Plan: npm linked to test manually

Reviewers: sbahra, jpihlaja, ahicks

Reviewed By: jpihlaja, ahicks

Maniphest Tasks: T21563

Differential Revision: https://hack.in.backtrace.io/D5369

lib/bpg.js

@@ -226,6 +226,8 @@ class BPG {
       throw Error('unknown action');
     }
 
+    if (typeof fields != 'object')
+      throw Error('fields must be a dict')
     /*
      * Construct key object from parent, otherwise construct from
      * child.