Fixed `_unquote_unreserved()` method in utils/url.
It was throwing exception when parsing wrongly encoded url.
Fixed `get_encoding_from_headers()`. When no charset was specified for
non-text content, it referenced undeclared `encoding` variable.
Fixed `get_encoding_from_headers()`. When empty encoding is specified,
return None instead of ''.
Fixed `get_html()` - only whitespace body is also considered as an empty
body now.