simplify memoization logic

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

lib/rhino/error.rb

@@ -14,39 +14,30 @@ module Rhino
 
     # most likely a Rhino::JS::JavaScriptException
     def cause
-      return @cause if defined?(@cause)
-      @cause = begin
-        if @native.respond_to?(:cause) && @native.cause
+      @cause ||= if @native.respond_to?(:cause) && @native.cause
         @native.cause
       else
         @native.is_a?(JS::RhinoException) ? @native : nil
       end
     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