improve performance of Message#find_attachment

[ahorek]

this is a rebased version of #825

benchmark

require 'mail'
require 'benchmark/ips'
part = Mail::Part.new.tap do |part|
  part.content_disposition = 'attachment; filename="test.asc"'
  part.content_type  = 'application/pgp-signature'
end

Benchmark.ips do |x|
  x.report('test find_attachment') { part.send(:find_attachment) }
end

result

patch:     25437.0 i/s
master:    13268.2 i/s - 1.92x  slower

[ahorek]

rebased