Render exceptions with multi-line messages like MRI

Fixes a test failure in rspec-mocks.

kernel/common/exception.rb

@@ -50,8 +50,14 @@ def awesome_backtrace
   end
 
   def render(header="An exception occurred", io=STDERR, color=true)
+    message_lines = message.to_s.split("\n")
+
     io.puts header
-    io.puts "    #{message} (#{self.class})"
+    io.puts "    #{message_lines.shift} (#{self.class})"
+
+    message_lines.each do |line|
+      io.puts "    #{line}"
+    end
 
     if @custom_backtrace
       io.puts "\nUser defined backtrace:"

spec/core/exception/render_spec.rb

@@ -0,0 +1,10 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+require 'stringio'
+
+describe "Exception#render" do
+  it "renders a multi-line message with the class name included in the first line" do
+    io = StringIO.new
+    Exception.new("line1\nline2\nline3").render("", io)
+    io.string.should include("line1 (Exception)\n")
+  end
+end