Add specs for subject prefix matching
user
```ruby
require 'spec_helper'

describe Mutant::Matcher::Compiler::SubjectPrefix do
  let(:object) { described_class.new(Mutant::Expression.parse('Foo*')) }

  let(:_subject) { double('Subject', expression: Mutant::Expression.parse(subject_expression)) }

  describe '#call' do
    subject { object.call(_subject) }

    context 'when subject expression is prefixed by expression' do
      let(:subject_expression) { 'Foo::Bar' }

      it { should be(true) }
    end

    context 'when subject expression is NOT prefixed by expression' do
      let(:subject_expression) { 'Bar' }

      it { should be(false) }
    end
  end
end
```
assistant