Never change the underlying string encoding

StringIO should be a view of given source string and
set_encoding shouldn't change source encoding.

Similar change for binmode and set_encoding_by_bom.

Author: jeremyevans

ext/stringio/stringio.c

@@ -278,9 +278,6 @@ set_encoding_by_bom(struct StringIO *ptr)
     if (idx) {
 	extenc = rb_enc_from_index(idx);
 	ptr->pos = bomlen;
-	if (ptr->flags & FMODE_WRITABLE) {
-	    rb_enc_associate_index(ptr->string, idx);
-	}
     }
     ptr->enc = extenc;
     return extenc;
@@ -696,9 +693,6 @@ strio_binmode(VALUE self)
     rb_encoding *enc = rb_ascii8bit_encoding();
 
     ptr->enc = enc;
-    if (WRITABLE(self)) {
-	rb_enc_associate(ptr->string, enc);
-    }
     return self;
 }
 
@@ -1859,9 +1853,6 @@ strio_set_encoding(int argc, VALUE *argv, VALUE self)
 	}
     }
     ptr->enc = enc;
-    if (!NIL_P(ptr->string) && WRITABLE(self)) {
-	rb_enc_associate(ptr->string, enc);
-    }
 
     return self;
 }

test/stringio/test_stringio.rb

@@ -304,7 +304,7 @@ def test_set_encoding
     f = StringIO.new()
     f.set_encoding("ISO-8859-16:ISO-8859-1")
     assert_equal(Encoding::ISO_8859_16, f.external_encoding)
-    assert_equal(Encoding::ISO_8859_16, f.string.encoding)
+    assert_equal(Encoding::UTF_8, f.string.encoding)
     assert_nil(f.internal_encoding)
   end