Don't modify $EASYRSA_EXTRA_EXTS in gen_req()

$EASYRSA_EXTRA_EXTS shouldn't be modified when creating certificate request, otherwise subsequent call to sign_req() will fail. This caused build-full-{client,server} to fail when --subject-alt-name is supplied.

Fixes issue #25.
Merge pull request #1 from SAPikachu/patch-1

Don't modify $EASYRSA_EXTRA_EXTS in gen_req()