Remove misleading frozen string check from proxy

Let the logic in method_double handle this like it does for other
classes.

lib/rspec/mocks/proxy.rb

@@ -35,14 +35,9 @@ def initialize(object, order_group, options={})
 
       # @private
       def ensure_can_be_proxied!(object)
-        return unless object.is_a?(Symbol) || (object.is_a?(String) && object.frozen?)
+        return unless object.is_a?(Symbol)
 
-        msg = "Cannot proxy frozen objects"
-        if Symbol === object
-          msg << ". Symbols such as #{object} cannot be mocked or stubbed."
-        else
-          msg << ", rspec-mocks relies on proxies for method stubbing and expectations."
-        end
+        msg = "Cannot proxy frozen objects. Symbols such as #{object} cannot be mocked or stubbed."
         raise ArgumentError, msg
       end
 

spec/rspec/mocks/space_spec.rb

@@ -240,14 +240,6 @@ def create_generations
       expect { space1.proxy_for(object) }.to raise_error(ArgumentError, expected_message)
     end
 
-    it 'raises ArgumentError with message if object is frozen' do
-      space1 = Space.new
-      object = "subject".freeze
-      expected_message = "Cannot proxy frozen objects, rspec-mocks relies on proxies for method stubbing and expectations."
-
-      expect { space1.proxy_for(object) }.to raise_error(ArgumentError, expected_message)
-    end
-
     def in_new_space_scope
       RSpec::Mocks.setup
       yield