Draft: Rewrite BeInstanceOfExpectation to match mspec behaviour

I don't really like the result. We should probably split up the matcher
and the expectation and create more generic code.

test/mspec/spec/matchers/be_an_instance_of_spec.rb

@@ -1,23 +1,6 @@
 require 'spec_helper'
 #require 'mspec/expectations/expectations'
 #require 'mspec/matchers'
-class BeAnInstanceOfMatcher
-  attr_reader :failure_message, :negative_failure_message
-
-  def initialize(klass)
-    @wrapped = BeInstanceOfExpectation.new(klass)
-    @klass = klass
-  end
-
-  def matches?(subject)
-    @wrapped.match(subject)
-    true
-  rescue SpecFailedException => e
-    @failure_message = ["Expected #{subject.inspect} (#{subject.class})", "to be an instance of #{@klass}"]
-    @negative_failure_message = ["Expected #{subject.inspect} (#{subject.class})", "not to be an instance of #{@klass}"]
-    nil
-  end
-end
 
 module BeAnInOfSpecs
   class A

test/support/spec.rb

@@ -567,17 +567,30 @@ def inverted_match(subject)
   end
 end
 
-class BeInstanceOfExpectation
+class BeAnInstanceOfMatcher
   def initialize(klass)
     @klass = klass
   end
 
   def match(subject)
-    raise SpecFailedException, "#{subject.inspect} (#{subject.class}) should be an instance of #{@klass}" if !subject.instance_of?(@klass)
+    raise SpecFailedException, failure_message.join(' ') unless matches?(subject)
   end
 
   def inverted_match(subject)
-    raise SpecFailedException, "#{subject.inspect} (#{subject.class}) should not be an instance of #{@klass}" if subject.instance_of?(@klass)
+    raise SpecFailedException, negative_failure_message.join(' ') if matched?(subject)
+  end
+
+  def matches?(subject)
+    @message_prelude = "Expected #{subject.inspect} (#{subject.class})"
+    subject.instance_of?(@klass)
+  end
+
+  def failure_message
+    [@message_prelude, "to be an instance of #{@klass}"]
+  end
+
+  def negative_failure_message
+    [@message_prelude, "not to be an instance of #{@klass}"]
   end
 end
 
@@ -1500,7 +1513,7 @@ def be_kind_of(klass)
   end
 
   def be_an_instance_of(klass)
-    BeInstanceOfExpectation.new(klass)
+    BeAnInstanceOfMatcher.new(klass)
   end
 
   def be_ancestor_of(klass)