net/http: fix typo in TestTransportReadToEndReusesConn
net/http: fix typo in TestTransportReadToEndReusesConn

The test sets a Content-Type where it looks like it wants a Content-Length. The test passes because the Content-Length header is automatically added anyway, but fix the typo and set Content-Length as intended.

Change-Id: Ic2af778f82c3e9d58e164892f6ac6ef5745f884f
Reviewed-on: https://go-review.googlesource.com/c/go/+/246977
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Trust: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go