Task's hours get restored when changing task description

[maszczyn]

Environment:
Redmine version 2.3.2.stable
Ruby version 1.9.3-p194 (2012-04-20) [x86_64-linux]
Rails version 3.2.13
Environment production
Database adapter Mysql2
Redmine plugins:
redmine_backlogs v1.0.5

I noticed that tasks with zero hours estimate get their estimate erroneously restored to the original value, in certain circumstances.

Steps to reproduce:

1. Create a new task with non-zero estimate, for example 5 hours
2. Open the task and change the estimate to 0
3. Open the task and change its description
4. Observe description change (as expected) AND the estimate reverted to original value of 5 hours (this is not expected)

It seems that leaving the "Remaining hours" field empty and saving causes the estimate to be reverted to the original value.