net/http/httputil: don't use DisableKeepAlives in DumpRequestOut, fix build

I broke the build in https://golang.org/change/207950a when I made
http.Transport send "Connection: close" request headers when
DisableKeepAlives was set true because I didn't run all the tests
before submitting.

httputil.DumpRequestOut used Transport to get its output, and used it
with DisableKeepAlives, so this changed the output.

Rather than updating golden data in our tests (because surely others
depend on the exact bytes from these in their tests), switch to not
using DisableKeepAlives in DumpRequestOut instead, so the output is
the same as before.

Change-Id: I9fad190be8032e55872e6947802055a6d65244df
Reviewed-on: https://go-review.googlesource.com/1632
Reviewed-by: Andrew Gerrand <adg@golang.org>