fix customised save format options never override default option

The default option DEFAULT_HTML is always being used despite of :save_with=>0. 

doc.to_html(:save_with => 0)  doesn't work because 
options[:save_with] |= save_option  #=> DEFAULT_HTML, which is 71. 

options[:save_with] = save_option unless options[:save_with] # this line does the job, it means when :save_with is set with some customized saving options, such as NO_DECLARATION | AS_HTML, then the default one should not be used anymore.