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")