fix(rails): fix nil dereference in SQL Active Record bind key

[tomlongridge]

Goal

Older versions of mysql2 adapter do not contain the bind key in the Active Record notification.

Changeset

Changed

• railtie.rb - added extra guard for key not being present in notification

Tests

I was unable to get a Rails instance with mysql2 v0.3.x working to reproduce this issue. However I have run it with later versions to ensure that we pick up the bind key correctly still (this is also checked on MazeRunner tests).

Linked issues

Fixes #555

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[tobyhs]

a wording suggestion, otherwise lgtm