Add X-Poedit-SearchPathExcluded-0 header support

As seen at https://github.com/robincornett/simple-listings-genesis/commit/ab3642f785f4324942aeb0323061bd8ace341812#diff-dfa28679ad06a5c3fe51c70a4b389d4fR27
Update unit tests for X-Poedit-SearchPathExcluded-0 header
Merge pull request #32 from GaryJones/search-path-exclude

Add X-Poedit-SearchPathExcluded-0 header support