Refactored strings::join to be more Pythonic.
Refactored strings::join to be more Pythonic.

Review: https://reviews.apache.org/r/7715

git-svn-id: 03d7800a49d8905ef4fbcc296184319e19243493@1402692 13f79535-47bb-0310-9956-ffa450edef68

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