Recalculate content-length

It is needed according to
https://github.com/django/django/blob/master/django/middleware/gzip.py#L46

pipeline/middleware.py

@@ -17,6 +17,7 @@ class MinifyHTMLMiddleware(object):
         if response.has_header('Content-Type') and 'text/html' in response['Content-Type']:
             try:
                 response.content = minify_html(response.content.strip())
+                response['Content-Length'] = str(len(response.content))
             except DjangoUnicodeDecodeError:
                 pass
         return response