Support using the HEAD request in the HTTP library

Support using the HEAD request in the HTTP library

There was a number of issues in relation to using the HTTP method HEAD

* The HTTP parser did not parse HEAD correctly
* HEAD respoonse could not have a content length and empty body
* The client could not receive a HEAD response with content length an empty body

I the server we support writing to the output stream when replying to a head request.

They should now be fixed and a test for HEAD have been added

R=ager@google.com

BUG=dart:3205
TEST=tests/standalone/io/http_head_test.dart, tests/standalone/io/http_parser_test.dart

Committed: https://code.google.com/p/dart/source/detail?r=7986

[Mads Ager (google), 2012-05-25 10:06:12]

lgtm