Fix connection when mysql2 v0.5.7

[KeiSAKATA]

Fix #96

mysql2 v0.5.7 removed :password from raw_connection.query_options, so this PR now derives connection parameters from ActiveRecord::Base.connection_db_config.configuration_hash (host/username/password/database) instead of query_options.

before

When I use database_rewinder with mysql2 v0.5.7.

$ bundle exec rspec spec/models/user_spec.rb

An error occurred in a `before(:suite)` hook.
Failure/Error: DatabaseRewinder.clean_with :truncation, except: %w[configurations containers server_clusters]

Mysql2::Error::ConnectionError:
  Access denied for user 'root'@'192.168.65.1' (using password: NO)
# ./spec/app_spec_helper.rb:12:in 'block (2 levels) in <main>'


Finished in 0.04335 seconds (files took 10.57 seconds to load)
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This PR

$ bundle exec rspec spec/models/user_spec.rb
..................

Finished in 2.25 seconds (files took 1.45 seconds to load)
18 examples, 0 failures