Make the argument of `Kernel#catch` optional as Ruby does

Also implement the following changes.

* Add tests
* Use `Object#equal?` to compare tags for Ruby compatibility
* Use `attr_reader`

mrbgems/mruby-catch/mrblib/catch.rb

 class ThrowCatchJump < Exception
+  attr_reader :_tag, :_val
   def initialize(tag, val)
-    @tag = tag
-    @val = val
+    @_tag = tag
+    @_val = val
     super("uncaught throw #{tag.inspect}")
   end
-  def _tag
-    @tag
-  end
-  def _val
-    @val
-  end
 end
 
 module Kernel
-  def catch(tag, &block)
+  def catch(tag=Object.new, &block)
     block.call(tag)
   rescue ThrowCatchJump => e
-    unless e._tag == tag
+    unless e._tag.equal?(tag)
       raise e
     end
     return e._val

mrbgems/mruby-catch/test/catch.rb

+assert "return throw value" do
+  val = ["val"]
+  result = catch :foo do
+    loop do
+      loop do
+        throw :foo, val
+        break
+      end
+      flunk("should not reach here")
+    end
+    false
+  end
+  assert_same(val, result)
+end
+
+assert "no throw" do
+  assert_equal(:foo, catch(:bar){:foo})
+end
+
+assert "no throw value" do
+  result = catch :foo do
+    throw :foo
+    1
+  end
+  assert_equal(nil, result)
+end
+
+assert "pass the given tag to block" do
+  tag = [:foo]
+  catch(tag){|t| assert_same(tag, t)}
+end
+
+assert "tag identity" do
+  assert_raise_with_message_pattern(Exception, "uncaught throw *") do
+    catch [:tag] do
+      throw [:tag]
+    end
+    flunk("should not reach here")
+  end
+end
+
+assert "without catch arguments" do
+  result = catch do |tag1|
+    catch do |tag2|
+      throw tag1, 1
+      flunk("should not reach here 1")
+    end
+    flunk("should not reach here 2")
+  end
+  assert_equal(1, result)
+end
+
+assert "catches across invocation boundaries" do
+  v = []
+  catch :one do
+    v << 1
+    catch :two do
+      v << 2
+      throw :one
+      v << 3
+    end
+    v << 4
+  end
+  assert_equal([1,2], v)
+end
+
+assert "catches in the nested invocation with the same key" do
+  v = []
+  catch :tag do
+    v << 1
+    catch :tag do
+      v << 2
+      throw :tag
+      v << 3
+    end
+    v << 4
+  end
+  assert_equal([1,2,4], v)
+end