Fixed undefined index in ::fromString method. refs#5608
Added unit tests
Merge branch 'hotfix/5773'

Close #5773
Fixes #5772
Merge pull request #5608 from mschindler83/zf5608

Http\Headers undefined index when calling ::fromString(' content-type: text/plain')
Merge branch 'hotfix/5608' into develop

Forward port #5608