add 'default' to the list of reserved keys & improve err msg.

lib/pry/config/behavior.rb

@@ -5,7 +5,7 @@ module Pry::Config::Behavior
                    "[]", "[]=", "merge!",
                    "respond_to?", "key?", "refresh",
                    "forget", "inherited_by", "to_h",
-                   "to_hash", "_dup"
+                   "to_hash", "_dup", "default"
                   ].freeze
 
   def self.included(klass)
@@ -39,7 +39,7 @@ module Pry::Config::Behavior
   def []=(key, value)
     key = key.to_s
     if RESERVED_KEYS.include?(key)
-      raise ArgumentError, "sorry, '#{key}' is a reserved configuration key."
+      raise ArgumentError, "few things are reserved by pry, but using '#{key}' as a configuration key is."
     end
     @writes[key] = value
   end

spec/config_spec.rb

@@ -2,7 +2,7 @@ require_relative 'helper'
 describe Pry::Config do
   describe "reserved keys" do
     it "raises an ArgumentError on assignment of a reserved key" do
-      local = Pry::Config.from_hash({})
+      local = Pry::Config.new
       Pry::Config::RESERVED_KEYS.each do |key|
         should.raise(ArgumentError) { local[key] = 1 }
       end