[ruby-core:83055] [Ruby trunk Bug#13949] String#unpack with 'M' directive can create strings with wrong code range

Issue #13949 has been reported by nirvdrum (Kevin Menard).

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Bug #13949: String#unpack with 'M' directive can create strings with wrong code range
https://bugs.ruby-lang.org/issues/13949

* Author: nirvdrum (Kevin Menard)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN
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I've noticed that String#unpack with the 'M' directive can create strings that should be CR_7BIT as CR_VALID. The issue appears to have been introduced in r30542, which assumes that all ASCII-8BIT strings must be CR_VALID. It's possible this was correct back during Ruby 1.9.3 development and just wasn't updated. I'm not familiar enough with the history to tell.

A simple reproduction showing the issue is:

```
res = '0123456789=\n'.unpack('M').first
p res
p res.encoding
p res.bytes
p res.ascii_only?

puts

packed = res.bytes.pack('c*')
p packed
p packed.encoding
p packed.bytes
p packed.ascii_only?

```

This yields the following output:

```
"0123456789=\\n"
#<Encoding:ASCII-8BIT>
[48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 61, 92, 110]
false

"0123456789=\\n"
#<Encoding:ASCII-8BIT>
[48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 61, 92, 110]
true
```

Both strings have exactly the same contents with the same encoding. But, depending on how you construct them, one is consider to be CR_7BIT (indicated by the String#ascii_only?) value, and one is considered to be CR_VALID. I believe CR_7BIT is the correct code range value in this situation.



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