Revert "(get) return undefined when key is expired"

This reverts commit dcb3a19.
This fixes #154.

_src/lib/node_cache.coffee

@@ -510,8 +510,8 @@ module.exports = class NodeCache extends EventEmitter
 		# data is invalid if the ttl is too old and is not 0
 		# console.log data.t < Date.now(), data.t, Date.now()
 		if data.t isnt 0 and data.t < Date.now()
-			_retval = false
 			if @options.deleteOnExpire
+				_retval = false
 				@del( key )
 			@emit( "expired", key, @_unwrap(data) )
 

_src/test/mocha_test.coffee

@@ -1014,11 +1014,11 @@ describe "`#{pkg.name}@#{pkg.version}` on `node@#{process.version}`", () ->
 				return
 
 
-			it "set a key key with a cache initialized with no automatic delete on expire should be undefined", (done) ->
+			it "set a key key with a cache initialized with no automatic delete on expire", (done) ->
 				localCacheNoDelete.set state.key1, state.val
 				setTimeout(() ->
 					res = localCacheNoDelete.get state.key1
-					should(res).eql(undefined)
+					should(res).eql(state.val)
 					done()
 					return
 				, 500)