fix #5065
add unit test for \Zend\Http\Client::prepareHeaders
add inline doc for \Zend\Http\Client::prepareHeaders
add missing use for Zend\Uri\Http
Merge pull request #5065 from ClemensSahs/hotfix/5042

[Zend\Http\Client] dupplicate header keys in prepareHeaders

Conflicts:
	tests/ZendTest/Http/ClientTest.php
Merge branch 'hotfix/5065'

Close #5065