net/http: make HTTP/1.x handlers able to sent undeclared trailers like HTTP/2

[bradfitz]

The HTTP/2 server permits setting Trailers mid-Handler without predeclaring them the "Trailer" header by setting "Trailer:Foo" = "Bar". Since a colon isn't allowed in a trailer, that magic prefix is safe.

Do the same for HTTP/1.x. (and thus also in httptest.ResponseRecorder).

[gopherbot]

CL https://golang.org/cl/32479 mentions this issue.