s3: Do not assert actual size in content-length for HEAD

A value of Content-Length returning with HEAD in net/http is now actual
content size from Go 1.2 but not Go 1.1.

See https://code.google.com/p/go/source/detail?r=3a388be1862ee310ec0036e3bc5bceeaf30ce315&path=/src/pkg/net/http/server.go

Fix to assert only its type to pass tests for 1.1 and 1.2.
WIP: Add travis-ci tweak