Allow indifferent access for url-based config

ConnectionPool.new is expecting to be able to access the config with
symbolized keys. Prior to this fix, this was causing any connection pool
settings to be ignored.

Most adapters avoid this issue by calling symbolize_keys before
attempting to connect, so the connection still works.
Merge pull request #274 from erykwalder/pool-config

Allow indifferent access for url-based config