[ruby/stringio] Fix ascii_only? flag in strio_write

(https://github.com/ruby/stringio/pull/77)

Followup of #79

`rb_str_resize()` was changed by b0b9f7201a  .

```c
rb_str_resize(string, shorter) // clear ENC_CODERANGE in some case
rb_str_resize(string, longer) // does not clear ENC_CODERANGE anymore
```

```c
// rb_str_resize in string.c
if (slen > len && ENC_CODERANGE(str) != ENC_CODERANGE_7BIT) {
  ENC_CODERANGE_CLEAR(str);
}
```

I think this change is based on an assumption that appending null bytes
will not change flag `ascii_only?`.

`strio_extend()` will make the string longer if needed, and update the
flags correctly for appending null bytes.
Before `memmove()`, we need to `rb_str_modify()` because updated flags are not
updated for `memmove()`.

https://github.com/ruby/stringio/commit/b31a538576

ext/stringio/stringio.c

@@ -915,9 +915,6 @@ strio_extend(struct StringIO *ptr, long pos, long len)
 	if (pos > olen)
 	    MEMZERO(RSTRING_PTR(ptr->string) + olen, char, pos - olen);
     }
-    else {
-	rb_str_modify(ptr->string);
-    }
 }
 
 /*
@@ -1464,14 +1461,9 @@ strio_write(VALUE self, VALUE str)
 	}
     }
     else {
-	int cr0 = ENC_CODERANGE(ptr->string);
-	int cr = ENC_CODERANGE_UNKNOWN;
-	if (rb_enc_asciicompat(enc) && rb_enc_asciicompat(enc2)) {
-	    cr = ENC_CODERANGE_AND(cr0, ENC_CODERANGE(str));
-	}
 	strio_extend(ptr, ptr->pos, len);
+	rb_str_modify(ptr->string);
 	memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
-	if (cr != cr0) ENC_CODERANGE_SET(ptr->string, cr);
     }
     RB_GC_GUARD(str);
     ptr->pos += len;

test/stringio/test_stringio.rb

@@ -961,6 +961,12 @@ class TestStringIO < Test::Unit::TestCase
     assert_predicate(s.string, :ascii_only?)
     s.write "\u{431 43e 433 443 441}"
     assert_not_predicate(s.string, :ascii_only?)
+
+    s = StringIO.new("\u{3042}")
+    s.rewind
+    assert_not_predicate(s.string, :ascii_only?)
+    s.write('aaaa')
+    assert_predicate(s.string, :ascii_only?)
   end
 
   def assert_string(content, encoding, str, mesg = nil)