Recommend "B" flag to Apache config

Summary: Split from D1921 via D1742. The "B" flag prevents excessive unescaping, especially of "+" into " ".

Test Plan: Added "B" to server config, var_dump()'d path with "+" in it, got "+" instead of " ".

Reviewers: nh, vrana, btrahan

Reviewed By: nh

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1922