Fixed a bug in the processing of HTTP response headers

Response header values with leading or trailing whitespace were parsed incorrectly, even though the HTTP standard specifies that such whitespace should be ignored (http://www.ietf.org/rfc/rfc2616.txt - Section 4.2).
This change trims said whitespace from all HTTP reponse header values.
add haxe.Utf8.sub tests
raise Sys_exit exception from Sys.exit call in interpreter to allow flushing context before actually terminating (was swallowing warnings in optimization unit tests)
fixed again #2555, this time correctly keep the private class path (a.b._C.Name becomes a.b.Name as if it was public)
Merge pull request #2525 from agersant/development

Fixed a bug in the processing of HTTP response headers
[cpp] Do not eliminate 'never' accessors unless they are also external