replace array_merge with manual iteration for performance
merged branch jimbojsb/master (PR #280)

This PR was merged into the master branch.

Commits
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37372cd replace array_merge with manual iteration for performance

Discussion
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Fix performance when tokenizing massive headers

I made this update to fix a performance issue when using very large headers. The use case is SendGrid's bulk messaging API in which there can be many thousands of lines of JSON in one  X-SMTPAPI header. array_merge() is known to be slow when dealing with arrays over about 100 items and being called repeatedly, because it's doing a key re-sort operation every time.

This change, in my 1000 item example, made the processing that happens inside ->send() go from 2 minutes to 2 seconds.
updated CHANGES