[Fix #345] Fix error of Rails/AfterCommitOverride on after_commitwith a lambda

[pocke]

Fixes #345

I got the same error on our Rails project, and I've confirmed the error is fixed by this patch.

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[pocke]

I guessed it also needs str_type? check, but it looks unnecessary.
I tried to define a callback with a string name, like after_commit 'foo' in a Rails app, but it raises an error during defining the callback. So we only need sym_type? check.

[koic]

Thanks!