add type check in Hash#merge!

mrbgems/mruby-hash-ext/mrblib/hash.rb

 class Hash
   def merge!(other, &block)
+    raise "can't convert argument into Hash" unless other.respond_to?(:to_hash)
     if block
       other.each_key{|k|
         self[k] = (self.has_key?(k))? block.call(k, self[k], other[k]): other[k]