Incorrect field #default_value handling

Problem/Motivation

The default value handling for the field widget form uses '#default_value' => (isset($items[0]) && isset($items[0]['value'])) ? $items[0]['value'] : 1, which causes the checkbox always being activated and preventing the field's default value being an unchecked state.

Proposed resolution

Use proper default value handling, using the field value, if present, and the field instance's configured default value: '#default_value' => (isset($items[$delta]) && isset($items[$delta]['value'])) ? $items[$delta]['value'] : $instance['default_value'][$delta],

Remaining tasks

Write a patch, review, commit.