Bug: some inappropriate behaviors in keyword arguments

[hachi8833]

They are edge cases when keyword args are not provided.

1:

def foo(a: 1)
  puts a
end

foo
#=> 1   # ok

foo a: 99
#=> 99  # ok

foo 99
#=> 1   # wrong: should be ArgumentError

2:

def foo(a, b:)
  puts a
  puts b
end

# ok
foo "bar", b: "baz"
#=> bar
#=> baz

# wrong
foo b: "baz"
#=> baz    # wrong: should also be ArgumentError, and unassigned variable 'a' should not be 'baz'
#=> baz