Fix ascii_only? flag in strio_write (#77)

Followup of #79
`rb_str_resize()` was changed by ruby/ruby@b0b9f72 .
```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()`.

Author: tompng

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 {
-intcr0=ENC_CODERANGE(ptr->string);
-intcr=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 @@ def test_coderange_after_overwrite
 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
 
 defassert_string(content,encoding,str,mesg=nil)