Re-add and deprecate default_engine

Closes #43.

Author: sferik

lib/multi_json.rb

@@ -19,6 +19,12 @@ def initialize(message, backtrace, data)
     ["json/pure", :json_pure]
   ]
 
+  # TODO: Remove for 2.0 release (but no sooner)
+  def default_engine
+    deprecate("MultiJson.default_engine is deprecated and will be removed in the next major version. Use MultiJson.default_adapter instead.")
+    self.default_adapter
+  end
+
   # The default adapter based on what you currently
   # have loaded and installed. First checks to see
   # if any adapters are already loaded, then checks

multi_json.gemspec

@@ -25,8 +25,11 @@ Gem::Specification.new do |gem|
   MultiJson.engine is deprecated and will be removed in the next major version.
   Use MultiJson.adapter instead.
 
-  MultiJson.engine= is deprecated and will be removed in the next major version.
-  Use MultiJson.use instead.
+  MultiJson.engine= is deprecated and will be removed in the next major
+  version. Use MultiJson.use instead.
+
+  MultiJson.default_engine is deprecated and will be removed in the next major
+  version. Use MultiJson.default_adapter instead.
 
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 eos