Parser emits that `false` is not allowed boolean type for default

[kylef]

The following API Blueprint emits the warning:

invalid value for 'boolean' type. allowed values are 'true' or 'false'

For the default value, where the default is false, same warning if it is wrapped in backticks or not. I expect it is not counting for the new line (\n) in the section. The same example using number type does work correctly.

FORMAT: 1A

# API Elements Data Structures

## Data Structures

### IsAdmin (boolean)

Whether the user is an admin

#### Default

`false`

[kylef]

Possibly related to https://github.com/apiaryio/drafter/issues/607