Fixed options[:temp_path] being deleted

In my project, we need to use a special writable partition and cannot use /tmp for writing. However, due to this statement inside pdf_from_html_file() and pdf_from_string():

temp_path = options.delete(:temp_path)

options[:temp_path] is dropped in pdf_from_string(). When options are passed from pdf_from_string() to pdf_from_html_file(), there is no more options[:temp_path]. So /tmp is always used in pdf_from_string().

This PR fixed options[:temp_path] being deleted. So custom path other than /tmp can be used  in pdf_from_string().