Improve space spec.

- The old spec only checked the number of proxies returned,
  and didn't actually check that it returned the right ones.
- The old spec only tested who were instances of the given
  class, and did not check instances of subclasses.

spec/rspec/mocks/space_spec.rb

@@ -6,16 +6,25 @@ module RSpec::Mocks
     describe "#proxies_of(klass)" do
       let(:space) { Space.new }
 
-      before do
-        space.proxy_for(Object.new)
-        space.proxy_for(Object.new)
-        space.proxy_for(Array.new)
-        space.proxy_for(String.new)
+      it 'returns proxies' do
+        space.proxy_for("")
+        expect(space.proxies_of(String).map(&:class)).to eq([PartialMockProxy])
       end
 
-      it 'returns the proxies for objects of the same type as klass' do
-        expect(space.proxies_of(String).size).to eq 1
-        expect(space.proxies_of(String)).to be_all { |obj| obj.is_a? RSpec::Mocks::PartialMockProxy }
+      it 'returns only the proxies whose object is an instance of the given class' do
+        grandparent_class = Class.new
+        parent_class      = Class.new(grandparent_class)
+        child_class       = Class.new(parent_class)
+
+        grandparent = grandparent_class.new
+        parent      = parent_class.new
+        child       = child_class.new
+
+        grandparent_proxy = space.proxy_for(grandparent)
+        parent_proxy      = space.proxy_for(parent)
+        child_proxy       = space.proxy_for(child)
+
+        expect(space.proxies_of(parent_class)).to match_array([parent_proxy, child_proxy])
       end
     end