Previously, golang.org/cl/75350 updated ReadMIMEHeader to ignore the
first header line when it begins with a leading space, as in the
following example:

GET / HTTP/1.1
  Host: foo.com
Accept-Encoding: gzip

However, golang.org/cl/75350 changed ReadMIMEHeader's behavior for the
following example: before the CL it returned an error, but after the
CL it ignored the first line.

GET / HTTP/1.1
  Host foo.com
Accept-Encoding: gzip

This change updates ReadMIMEHeader to always fail when the first header
line starts with a space. During the discussion for golang.org/cl/75350,
we realized we had three competing needs:

1. HTTP clients should accept malformed response headers when possible
   (ignoring the malformed lines).

2. HTTP servers should reject all malformed request headers.

3. The net/textproto package is used by multiple protocols (most notably,
   HTTP and SMTP) which have slightly different parsing semantics. This
   complicates changes to net/textproto.

We weren't sure how to best fix net/textproto without an API change, but
it is too late for API changes in Go 1.10. We decided to ignore initial
lines that begin with spaces, thinking that would have the least impact on
existing users -- malformed headers would continue to parse, but the
initial lines would be ignored. Instead, golang.org/cl/75350 actually
changed ReadMIMEHeader to succeed in cases where it previously failed
(as in the above example).

Reconsidering the above two examples, there does not seem to be a good
argument to silently ignore ` Host: foo.com` but fail on ` Host foo.com`.
Hence, this change fails for *all* headers where the initial line begins
with a space.

Updates #22464

Change-Id: I68d3d190489c350b0bc1549735bf6593fe11a94c
Reviewed-on: https://go-review.googlesource.com/80055
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>