In tempurl middleware, hmac uses the value of account metadata to
generate HMAC-SHA1 signature and hmac must accept a str-type string, not
a unicode string. The meta dict returned from get_info stroges special
chars as unicode however. So just encode it for tempurl using.
Closes-Bug: #1242644
Change-Id: I4be62eea014a573efc4748470de57dccf00e431d
abcecd26a7