Fix exhausted connection throw tag

[thehanslevi]

Fixes #2.

Summary

• corrected the connection-window drain throw tag from :CONNECTION_EXHAUSED to :CONNECTION_EXHAUSTED
• added a focused test/unit regression for a queued DATA frame that remains blocked after a connection-level WINDOW_UPDATE

Validation

• ruby test/test_http_agent.rb
• ruby test/test_encoding.rb
• ruby test/test_huffman_codes.rb
• rg -n "CONNECTION_EXHAUS" lib/ruby-h2/http-agent.rb test/test_http_agent.rb
• git diff --check