to_binary_s should always return ASCII-8BIT encoded strings

[tenderlove]

StringIO in Ruby <= 2.6.X didn't respect Encoding.default_internal, so
calling read on the StringIO wouldn't tag the string with the same
encoding. Ruby 2.7 fixes this bug. The problem is that to_binary_s
is returning a binary string and should always return an ASCII-8BIT
encoded string.

This patch changes the method to always return the ASCII-8BIT string
regardless of the value of Encoding.default_internal.

Here is the behavior of StringIO on Ruby 2.6:

$ ruby -v -rstringio -e'Encoding.default_internal = Encoding::UTF_8; io = StringIO.new("foo".force_encoding("ASCII-8BIT")); io.rewind; p io.read.encoding'
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin19]
-e:1: warning: setting Encoding.default_internal
 #<Encoding:ASCII-8BIT>

Here it is on 2.7:

$ ruby -v -rstringio -e'Encoding.default_internal = Encoding::UTF_8; io = StringIO.new("foo".force_encoding("ASCII-8BIT")); io.rewind; p io.read.encoding'
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-darwin19]
-e:1: warning: setting Encoding.default_internal
  #<Encoding:UTF-8>

We can see 2.7 respects default_internal, but unfortunately that means
that to_binary_s would return the wrong encoding.

[coveralls]

Coverage decreased (-2.0e-05%) to 99.91% when pulling 26d35a1 on tenderlove:always-return-ascii-8bit into 2f22031 on dmendel:master.

[dmendel]

Thanks! New version of BinData with this fix will be out shortly