fix travis build status image src of release/3.0 branch
refactor session store initializer
set the path attribute of the session cookie when a relative url root is set
config can contain an empty string. use presence to convert that to nil. pass it directly to :path. rails handles the nil case with its default like before.
Merge pull request #1120 from marutosi/finn-rel-3.0-readme-src

fix travis build status image src of release/3.0 branch
infer content type from contents of file instead of its file extension
* defaults to application/binary as its more secure than octet-stream
* set nosniff header to instruct browsers not to guess content type