Bypass RSpec::Mocks::Syntax when mass-assigning stubs via double().

When mock_with's `config.syntax = [:expect]`, the `#stub` method is not
available on the TestDouble.

This change bypasses the pluggable syntax layer, directly setting up
allowance for a Matcher::Receive instance.

Another option would be to use AcceptanceTarget.new(self).to(matcher)
however the `#to` DSL-like method feels out of place when not used with
the `allow(target)` receiver syntax, and also the AcceptanceTarget
delegation implementation complexity is not required for internals like
this.
Spec double() stubs hash in different syntax modes.

This tests a regression referenced in #306 where disabling the :should
syntax raised errors when a stubs hash was passed to double().
Add a changelog entry for #306.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>

Conflicts:
	Changelog.md