Handle problem when payload is a string.
Handle problem when payload is a string.

In particular when determining whether a pull request is from the
same repository.

GistIcon/travis-core,Tiger66639/travis-core,developerfm/travis-core,kidaa/travis-core,developerfm/travis-core,final-ci/travis-core,developerfm/travis-core,final-ci/travis-core,jantman/travis-core,travis-ci/travis-core,final-ci/travis-core,jantman/travis-core,GistIcon/travis-core,travis-ci/travis-core,kidaa/travis-core,Tiger66639/travis-core,kidaa/travis-core,Tiger66639/travis-core,travis-ci/travis-core,GistIcon/travis-core