Integration tests prefixing headers with "HTTP-"

When I pass HTTP headers like “Accept” or “If-Not-Modifed-Since” to a
get request in a Rails integration test, the test base class prefixes
the headers with “HTTP-”, and converts them to upper case.

Why does it prepend “HTTP-”? I looked over the spec at W3C, and there
is no mention of such a prefix. My browser sends “If-Not-Modified-
Since”, without a prefix.