[release-branch.go1.8] encoding/xml: disable checking of attribute syntax, like Go 1.7

[release-branch.go1.8] encoding/xml: disable checking of attribute syntax, like Go 1.7

Consider this struct, which expects an attribute A and a child C both ints:

    type X struct {
        XMLName xml.Name `xml:"X"`
        A       int      `xml:",attr"`
        C       int
    }

Go 1.2 through Go 1.7 were consistent: attributes unchecked,
children strictly checked:

    $ go1.7 run /tmp/x.go
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ok
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

Go 1.8 made attributes strictly checked, matching children:

    $ go1.8 run /tmp/x.go
    <X></X>              ok
    <X A=""></X>         ERROR strconv.ParseInt: parsing "": invalid syntax
    <X A="bad"></X>      ERROR strconv.ParseInt: parsing "bad": invalid syntax
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

but this broke XML code that had empty attributes (#19333).

In Go 1.9 we plan to start allowing empty children (#13417).
The fix for that will also make empty attributes work again:

    $ go run /tmp/x.go  # Go 1.9 development
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ERROR strconv.ParseInt: parsing "bad": invalid syntax
    <X></X>              ok
    <X><C></C></X>       ok
    <X><C/></X>          ok
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

For Go 1.8.1, we want to restore the empty attribute behavior
to match Go 1.7 but not yet change the child behavior as planned for Go 1.9,
since that change hasn't been through release testing.

Instead, restore the more lax Go 1.7 behavior, so that XML files
with empty attributes will not be broken until Go 1.9:

    $ go run /tmp/x.go  # after this CL
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ok
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

Fixes #19333.

Change-Id: I3d38ebd2509f5b6ea3fd4856327f887f9a1a8085
Reviewed-on: https://go-review.googlesource.com/39607
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Sarah Adams <shadams@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>