strio_read: preserve buffer encoding on partial reads (#95)

[[Bug #20418]](https://bugs.ruby-lang.org/issues/20418)
Ruby IO#read preserves the encoding on partial read, but change it when
reading the whole IO
from commit ruby/ruby@0ca7036:
> * io.c (read_all): should associate default external encoding.
> * io.c (io_read): should NOT associate default external encoding.
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

Author: casperisfine

ext/stringio/stringio.c

@@ -1623,10 +1623,9 @@ strio_read(int argc, VALUE *argv, VALUE self)
 if (len>rest) len=rest;
 rb_str_resize(str, len);
 MEMCPY(RSTRING_PTR(str), RSTRING_PTR(ptr->string) +ptr->pos, char, len);
-if (binary)
-rb_enc_associate(str, rb_ascii8bit_encoding());
-else
+if (!binary) {
 rb_enc_copy(str, ptr->string);
+	}
  }
 ptr->pos+=RSTRING_LEN(str);
 returnstr;

test/stringio/test_stringio.rb

@@ -700,6 +700,18 @@ def test_read
 s.force_encoding(Encoding::US_ASCII)
 assert_same(s,f.read(nil,s))
 assert_string("",Encoding::UTF_8,s,bug13806)
+
+bug20418='[Bug #20418] ™€®'.b
+f=StringIO.new(bug20418)
+s=""
+assert_equal(Encoding::UTF_8,s.encoding,bug20418)
+f.read(4,s)
+assert_equal(Encoding::UTF_8,s.encoding,bug20418)
+
+f.rewind
+s=""
+f.read(nil,s)
+assert_equal(Encoding::ASCII_8BIT,s.encoding,bug20418)
 end
 
 deftest_readpartial
@@ -711,8 +723,8 @@ def test_readpartial
 assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT),f.readpartial(f.size))
 f.rewind
 # not empty buffer
-s='0123456789'
-assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT),f.readpartial(f.size,s))
+s='0123456789'.b
+assert_equal("\u3042\u3044".b,f.readpartial(f.size,s))
 end
 
 deftest_read_nonblock
@@ -736,8 +748,8 @@ def test_read_nonblock_no_exceptions
 assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT),f.read_nonblock(f.size))
 f.rewind
 # not empty buffer
-s='0123456789'
-assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT),f.read_nonblock(f.size,s))
+s='0123456789'.b
+assert_equal("\u3042\u3044".b,f.read_nonblock(f.size,s))
 end
 
 deftest_sysread