simplify memoization logic

replace some `if defined?` statements with a 
memoized if-statement.

lib/rhino/error.rb

@@ -1,55 +1,46 @@
 module Rhino
 
   class JSError < StandardError
-    
+
     def initialize(native)
       @native = native # NativeException wrapping a Java Throwable
       message = value ? value : ( cause ? cause.details : @native )
       super(message)
     end
-    
+
     def inspect
       "#<#{self.class.name}: #{message}>"
     end
-    
+
     # most likely a Rhino::JS::JavaScriptException
     def cause
-      return @cause if defined?(@cause)
-      @cause = begin
-        if @native.respond_to?(:cause) && @native.cause
-          @native.cause
-        else
-          @native.is_a?(JS::RhinoException) ? @native : nil
-        end
+      @cause ||= if @native.respond_to?(:cause) && @native.cause
+        @native.cause
+      else
+        @native.is_a?(JS::RhinoException) ? @native : nil
       end
     end
 
     def value
-      return @value if defined?(@value)
-      if cause.respond_to?(:value) # e.g. JavaScriptException.getValue
-        @value = cause.value
+      @value ||= if cause.respond_to?(:value) # e.g. JavaScriptException.getValue
+        cause.value
       elsif ( unwrap = self.unwrap ) && unwrap.respond_to?(:value)
-        @value = unwrap.value
-      else
-        @value = nil
+        unwrap.value
       end
     end
-    
+
     def unwrap
-      return @unwrap if defined?(@unwrap)
       cause = self.cause
-      if cause && cause.is_a?(JS::WrappedException) 
+      @unwrap ||= if cause && cause.is_a?(JS::WrappedException)
         e = cause.getWrappedException
         if e && e.is_a?(Java::OrgJrubyExceptions::RaiseException)
-          @unwrap = e.getException
+          e.getException
         else
-          @unwrap = e
+          e
         end
-      else
-        @unwrap = nil
       end
     end
-    
+
     def backtrace
       if js_backtrace = javascript_backtrace
         js_backtrace.push(*super)
@@ -57,7 +48,7 @@ module Rhino
         super
       end
     end
-    
+
     def javascript_backtrace(keep_elements = false)
       if cause.is_a?(JS::RhinoException)
         cause.getScriptStack.map do |element| # ScriptStackElement[]
@@ -67,9 +58,9 @@ module Rhino
         nil
       end
     end
-    
+
     Rhino::JS::RhinoException.useMozillaStackStyle(false)
-    
+
   end
-  
+
 end