test for inconsistent validation of Bool type

issue #16
Make 'Bool' type Moose compat.

Moose's 'Bool' definition is defined in
M::Util::TypeConstraints::Builtins.

    sub { !defined($_) || $_ eq "" || "$_" eq '1' || "$_" eq '0' }

But Mouse's XS implementation doesn't do the stringification.
Then, Moose accepts JSON::PP::Boolean as 'Bool' type(JSON::PP::Boolean
oveloads stringification operator). But Mouse doesn't.
This patch fix this incompatible behavior.
Merge pull request #24 from tokuhirom/json-boolean

Make 'Bool' type Moose compatible.