http: avoid frozen string bug in filter_body

Our rb_str_modify() became no-ops due to incomplete reverts
of workarounds for old Rubinius, causing rb_str_set_len to
fail with: can't set length of shared string (RuntimeError)

This bug was introduced due to improper workarounds for old
versions of Rubinius in 2009 and 2010:

commit 5e8979ad38efdc4de3a69cc53aea33710d478406
("http: cleanups for latest Rubinius")
commit f37c23704cb73d57e9e478295d1641df1d9104c7
("http: no-op rb_str_modify() for Rubies without it")
unicorn 4.6.2 - HTTP parser fix for Rainbows!

This release fixes a bug in Unicorn::HttpParser#filter_body
which affected some configurations of Rainbows!  There is
also a minor size reduction in the DSO.