hash each args is not compatible

Hi AJ!

I want to async enumeration with hash.
But {}.async(number).each args and {}.async(nil).each args are not compatible.
In addition, a FiberError often raises when enumerate a hash.

    h = {a: 123, b: 456}
    
    h.async.each {|item| p item}
    # => [:a, 123]
    # => [:b, 456]
    
    h.async(2).each {|item| p item}
    # => :a
    # => :b

    # A FiberError often raises.
    # /path/to/async_enum/lib/enumerator/async.rb:45:in `next': fiber called across threads (FiberError)
    #     from /path/to/async_enum/lib/enumerator/async.rb:45:in `block (3 levels) in each'
    #     from /path/to/async_enum/lib/enumerator/async.rb:44:in `loop'
    #     from /path/to/async_enum/lib/enumerator/async.rb:44:in `block (2 levels) in each'

So I try to fix.

Thanks.