Zend\Http: Unit tests for multi-line headers
Merge pull request #5916 from ralphschindler/hotfix/http-multiline-headers

Zend\Http: Unit tests for multi-line headers
[#5616] Revert code from #5608

Based on new tests and links to relevant specifications, the code from #5608
was invalid; header lines may never start with whitespace.

This patch reverts that code, and updates the `fromString()` logic to
capture whitespace on subsequent lines to include in the multi-line
header value. Tests have been updated where necessary to use a regex for
testing the existence of whitespace between multiline values.
Merge branch 'hotfix/5616'

Close #5616