…file (#288)

Fix GH-287

## Why?
If `@source = nil` is set at the end of the file, prevent
`IOSource#read_until` from raising an error.

> /.gems/gems/rexml-3.4.2/lib/rexml/parsers/baseparser.rb:524:in
'REXML::Parsers::BaseParser#pull_event': #<NoMethodError: undefined
method 'eof?' for nil> (REXML::ParseException)
> .gems/gems/rexml-3.4.2/lib/rexml/source.rb:275:in
'REXML::IOSource#read_until'


https://github.com/ruby/rexml/blob/185bdc737da406ba4f9564726849ad3477858eb2/lib/rexml/source.rb#L266-L276

1. Reaching the end of a file in `IOSource#read`.
2. If the string remains in `@scanner.rest`, use
`@scanner.scan_until(pattern)` to return `str`.
3. If `@source` is `nil`, an exception occurs.

… so the test is skipped (#292)

## Why?

JRuby's `Zlib::GzipReader#readline` does not support arguments, so when
using `Zlib::GzipReader.open`, it always returns an empty string and
does not function. We will temporarily exclude `Zlib::GzipReader` from
test coverage.

```
$ test/run.rb --location test/test_order.rb:49

Failure: test_more_ordering(REXMLTests::OrderTester)
/Users/naitoh/ghq/github.com/naitoh/rexml/test/test_order.rb:110:in `test_more_ordering'
     107:          count += 1
     108:        }
     109:
  => 110:        assert_equal( actual.size, count )
     111:      end if defined?(Zlib::GzipReader)
     112:   end
     113: end
<46> expected but was
<0>
```

See: jruby/jruby#8997

## Why?

GitHub: fix #289

We must reject all well-formed XMLs:

https://www.w3.org/TR/2006/REC-xml11-20060816/#proc-types

> Validating and non-validating processors alike MUST report violations
of this specification's well-formedness constraints in the content of
the [document
entity](https://www.w3.org/TR/2006/REC-xml11-20060816/#dt-docent) and
any other [parsed
entities](https://www.w3.org/TR/2006/REC-xml11-20060816/#dt-parsedent)
that they read.

No root element XML is not well-formed because `document` requires one
`element`:

https://www.w3.org/TR/2006/REC-xml11-20060816/#sec-well-formed

> [1] document ::= (
[prolog](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-prolog)
[element](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-element)
[Misc](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-Misc)* ) - (
[Char](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-Char)*
[RestrictedChar](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-RestrictedChar)
[Char](https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-Char)* )

## Why?

GitHub: Fix GH-296

XML without a root element is invalid XML, but to maintain backward
compatibility,
we will change the behavior of `REXML::Document.new("")` to be the same
as `REXML::Document.new(nil)`.

Reported by Joe Rafaniello. Thanks!!!