epub converter: fix <pre> 3

src/html_epub_preprocessor.py

@@ -406,6 +406,7 @@ def preprocess_pre_tags(chapter_tag):
             if isinstance(child, NavigableString):
                 text = escape(pre.text)
                 text = text.replace('\t', '      ')
+                text = text.replace(' ', '  ')
                 elements = re.split('\r\n|\n|\r', text)
                 for i in elements:
                     new_tag.append(NavigableString(i))