Accept range end position which exceededs the resource size

[kuenishi]

RFC 2616 14.35.1 Byte Ranges

If the last-byte-pos value is absent, or if the value is greater than
or equal to the current length of the entity-body, last-byte-pos is
taken to be equal to one less than the current length of the entity-
body in bytes.

This work is originally done by @shino and feedback of basho/mochiweb#17 and basho/mochiweb#7 .

[kuenishi]

We also removed the head

parse_range_request("bytes=0-") ->
    undefined;

to avoid handling special case undefined for our convenience. If this breaks something else, I can get rid of this.

[etrepum]

This had a test failure that somehow didn't show up when I was merging the request. Fixing it now. https://travis-ci.org/mochi/mochiweb/builds/47273580