Fix Boolean options with non boolean defaults

[elas7]

Inspired by this issue in minimist: https://github.com/substack/minimist/issues/44

It you have a boolean flag with a default to something that isn't a boolean. For example:

var parsed = yargs(process.argv.slice(2), {
  boolean: ['foo'],
  default: {foo: null}
})

Then calling with --foo will result in {"_": [], "foo": [null, true]}.

I updated setKey() to avoid creating an array if the key in argv is in flags.bools. Previously, it only checked if the current value for the key in argv was a Boolean, but that didn't consider the case when a non-Boolean is set by default.

Note that setting a non-Boolean default for a Boolean key can be useful, as in the referenced issue, to find out if a key was explicitly set by the user in the CLI.

[coveralls]

Coverage remained the same at 100.0% when pulling 8c805bb on elas7:fix-boolean into 48b1e6a on yargs:master.