code: def []=(key, value)
        case value
        when String
          string(key, equal_to: value.to_s)
        else
          any(key, equal_to: value.to_s)
        end
      end
docs: Shortcut to say a schema +key+ must be equal to a given +value+. When it
   does not recognize the value type it creates a "any" schema.
  
   Example:
     HashSchema.define do |s|
       s["a_string"] = "value"       # equivalent to: s.string("a_string", equal_to: "value")
       s["a_key"] = 0..5             # equivalent to: s.any("a_key", equal_to: "0..5")
     end