Fixit : Add HTTP_RANGE compatibility for ContentStore file streaming

1) I removed the test "content.length > 1048576" as I was not really
sure if it was useful or not.
As content.stream_data() is used for every request, my modified
streaming should also work.
I don't really understand what the comment in the caching section means
`# since we fetched it from DB, let's cache it going forward, but only
if it's < 1MB
# this is because I haven't been able to find a means to
stream data out of memcached`

I also removed all value tests from stream_data_in_range, as you said
that we shouldn't call the function with bad parameters.
I added `if 0 <= first <= last < content.length:` in middleware, which
is a short and easy to understand test.