= StaticHash

A StaticHash is simply a Hash that can only be assigned
once per key. Once assigned a subsequent attempt to assign
a value to the same key will raise an ArgumentError.

  h = StaticHash.new

  h["x"] = 1

  expect ArgumentError do
    h["x"] = 2
  end

The same error will be raised when using #update or #merge!.

  expect ArgumentError do
    h.update( "x"=>3 )
  end

