Fix boolean values according to DER specs

In BER encoding, any boolean with a non-zero value is considered as
TRUE. However, DER encoding require a value of 255 (0xFF) for TRUE.

This commit makes `mbedtls_asn1_write_bool` function uses `255` instead
of `1` for BOOLEAN values.

With this fix, boolean values are now reconized by OS X keychain (tested
on OS X 10.11).

Fixes #318.

library/asn1write.c

@@ -191,7 +191,7 @@ int mbedtls_asn1_write_bool( unsigned char **p, unsigned char *start, int boolea
     if( *p - start < 1 )
         return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
 
-    *--(*p) = (boolean) ? 1 : 0;
+    *--(*p) = (boolean) ? 255 : 0;
     len++;
 
     MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( p, start, len ) );