Use should be_nan syntax in tests

Author: rsim

spec/mondrian_spec.rb

@@ -689,7 +689,7 @@ def nonempty_crossjoin_from_cube(cube_name)
           MDX
         ).
         columns('[Measures].[LinRegR2]').execute
-      result.values.first.nan?.should == true
+      result.values.first.should be_nan
     end
   end
 
@@ -781,7 +781,7 @@ def nonempty_crossjoin_from_cube(cube_name)
           MDX
         ).
         columns('[Measures].[LinRegVariance]').execute
-      result.values.first.nan?.should == true
+      result.values.first.should be_nan
     end
   end