improve error message when a schema that is not a hash is passed to Schema#initialize

Author: notEthan

lib/json-schema/schema.rb

@@ -6,6 +6,8 @@ class Schema
     attr_accessor :schema, :uri, :validator
 
     def initialize(schema,uri,parent_validator=nil)
+      raise ArgumentError, "schema must be a hash; got: #{schema.inspect}" unless schema.is_a?(Hash)
+
       @schema = schema
       @uri = uri