Pass hostname to web socket instead of hardcoded localhost

[mspaulding06]

When attempting to use the playground with an IP other than "localhost" or "0.0.0.0" the output is unable to update. The following error is thrown when clicking the "Show Output" button in the interface.

Unhandled exception: Error connecting to 'localhost:8080': Connection refused (Socket::ConnectError)
  from /usr/share/crystal/src/socket/addrinfo.cr:73:15 in 'initialize'
  from /usr/share/crystal/src/socket/tcp_socket.cr:27:3 in 'initialize'
  from /usr/share/crystal/src/socket/tcp_socket.cr:27:3 in 'new'
  from /usr/share/crystal/src/http/web_socket/protocol.cr:275:5 in 'new'
  from /usr/share/crystal/src/http/web_socket/protocol.cr:323:14 in 'new'
  from /usr/share/crystal/src/http/web_socket.cr:35:5 in 'new'
  from /usr/share/crystal/src/http/web_socket.cr:34:3 in 'new'
  from /usr/share/crystal/src/compiler/crystal/tools/playground/agent.cr:10:11 in 'initialize'
  from /usr/share/crystal/src/compiler/crystal/tools/playground/agent.cr:9:3 in 'new'
  from /usr/share/crystal/src/gc/boehm.cr:164:23 in '~Crystal::Playground::Agent::instance:init'
  from /play:1:6 in '__crystal_main'
  from /usr/share/crystal/src/crystal/main.cr:110:5 in 'main_user_code'
  from /usr/share/crystal/src/crystal/main.cr:96:7 in 'main'
  from /usr/share/crystal/src/crystal/main.cr:119:3 in 'main'
  from __libc_start_main
  from _start
  from ???

exit status: 1

This is a small change that passes the hostname to the session so that it can be used for the web socket instead of the hardcoded "localhost".

[Sija]

This default should be set within the initializer.

[mspaulding06]

Looks like some of the tests have failed because they were unable to communicate with Docker's servers. I don't think I have any way to trigger them to run again.

[HertzDevil]

#12356 is probably a better fix than this PR.

[HertzDevil]

Duplicate of #12356