Improve performance of Message#find_attachment

[bpot]

This is faster because we,

• Don't trigger exceptions for flow control.
• Retrieve each header only once (at most).

The exceptions this was throwing were particularly expensive for us on jruby. This change cut our mail processing time in half for a particular task.

The tests are passing with this change but it's possible that the rescues in the previous code were catching other exceptions that will now bubble up. We could add a rescue to find_attachment_filename to catch those.

[grosser]

:+1: causing and catching exceptions is super-messy

@jeremy @bf4

[bpot]

@grosser cleaned up the duplication in #find_attachment_filename. I like to avoid using #send if possible.

[grosser]

@jeremy @bf4

[bf4]

Is there a benchmark you used that we can run to test against regressions?

[bpot]

@bf4 I don't have anything that's easily extractable, I was profiling an application.

I agree a benchmark suite would be really useful.