Fix bug; sometimes missing nginx variable

Fix bug; mruby_set ignore "cache" option

Suppress variable not found error because it is too verbose.

Implement Var#exist?

Var#exist? returns true when the nginx variable is found,
otherwise false.

Copy string object to nginx string

sometimes missing string object #125

Add Nginx::Request#sub_request?

Add Nginx::Upstream class and method

Fix bug; sometimes headers line include dust char

Support custom built-in header like "Server" header on output filter phase

Support controling response headers at output_filter phase

Allow deleting headers, creating new incoming headers

Fix memory leak when using mruby_set{,_code} directive

Fix memory leak when ngx_mruby raise error and return 5xx status code

add Dockerfile.onbuild for docker users' convenience.

Now you can build 'onbuild' docker image by
$ docker buld myname/ngx-mruby:onbuild - < Dockerfi...