Transaction API does not match documentation

[andyfangdz]

[REQUIRED] Step 2: Describe your environment

• Operating System version: macOS
• Firebase SDK version: 2.2.0
• Firebase Product: database (auth, database, storage, etc)

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

1. Create a database in firebase like

{
  "jobs": {
    "count": 0
  }
}

2. in Python, do root.child('jobs/count').transaction(lambda x: x + 1 if x else 1)

Raised error: ValueError: Value argument must be a non-empty dictionary.

Relevant Code:

firebase-admin-python/firebase_admin/db.py

Lines 204 to 207 in 99c48ef

	if not value or not isinstance(value, dict):
	raise ValueError('Value argument must be a non-empty dictionary.')
	if None in value.keys() or None in value.values():
	raise ValueError('Dictionary must not contain None keys or values.')

These lines of code assume that the value passed in is a dictionary, which was not mentioned in the documentation (https://firebase.google.com/docs/database/admin/save-data#section-transactions) at all.