Fix SVG attributes preserve aspect ratio and view box

[ashadulhoque]

lxml processing at class EpubHtml converts preserveAspectRatio and viewBox svg attributes to lowercase which generates epub validation error and resultant epub cover doesn't cover full page as it should. This hack restores the case of these attributes.

[james-c-barnes]

Just ran into this bug; 'viewBox' is renamed to 'viewbox'. Code hack works. I needed to convert the strings to bytes

        # https://github.com/aerkalov/ebooklib/pull/170
        tree_str = tree_str.replace(b'preserveaspectratio',b'preserveAspectRatio')
        tree_str = tree_str.replace(b'viewbox',b'viewBox')