Add specs for rb_interned_str and rb_interned_str_cstr

[herwinw]

Keeping this as a draft for now. The comment in the MRI source for rb_interned_str reads as follows:

/**
 * Identical to rb_str_new(), except it returns an infamous "f"string.  What is
 * a  fstring?  Well  it is  a special  subkind of  strings that  is immutable,
 * deduped globally, and managed by our GC.   It is much like a Symbol (in fact
 * Symbols  are dynamic  these days  and are  backended using  fstrings).  This
 * concept has been  silently introduced at some point in  2.x era.  Since then
 * it  gained  wider acceptance  in  the  core.   Starting from  3.x  extension
 * libraries can also generate ones.
 *
 * @param[in]  ptr           A memory region of `len` bytes length.
 * @param[in]  len           Length  of  `ptr`,  in bytes,  not  including  the
 *                           terminating NUL character.
 * @exception  rb_eArgError  `len` is negative.
 * @return     A  found or  created instance  of ::rb_cString,  of `len`  bytes
 *             length, of  "binary" encoding,  whose contents are  identical to
 *             that of `ptr`.
 * @pre        At  least  `len` bytes  of  continuous  memory region  shall  be
 *             accessible via `ptr`.
 */
VALUE rb_interned_str(const char *ptr, long len);

But the encoding of the result is always Encoding::US_ASCII, which leads to the following spec:

it "support binary strings that are invalid in ASCII encoding" do
  str = "foo\x81bar\x82baz".b
  result = @s.rb_interned_str(str, str.bytesize)
  result.encoding.should == Encoding::US_ASCII
  result.should == str.dup.force_encoding(Encoding::US_ASCII)
  result.should_not.valid_encoding?
end

I will create an issue for Ruby to get clarification about the desired behaviour.

[herwinw]

Upstream: https://bugs.ruby-lang.org/issues/21842

[herwinw]

Updated the specs to match the behaviour of ruby/ruby#15888.

This might not be the final version, ruby/ruby#15894 has a proposed change to return US-ASCII if everything is ASCII compatible, and BINARY otherwise.

[herwinw]

Updated again to match the new behaviour of ruby/ruby#15894

[eregon]

This description/test is confusing because the encoding is ignored by that function, it only receives char* and length.

[eregon]

Same

[eregon]

Looks great, just 2 comments