add method to Zend\Http\Response\Stream

the content length is now exposed with getters and setters
add method to Zend\Http\Response\Stream

the content length is now exposed with getters and setters
fixed phpdoc
change atrribute $this->contentlength to getContentLength()
Merge branch 'stream' of git://github.com/prolic/zf2 into hotfix/3498
[#3498] Minor logic changes

- Cache return of method call -- prevents multiple fetches
- use "null !==" instead of "!is_null" (internal consistency)
Merge branch 'hotfix/3498' into develop

Close #3498