Minor render issue with alpha filter (IE) and numbers

This is related to #550

input

```
a {
  filter: alpha(opacity=.3); }

div {
  filter: alpha(opacity=0.7); }
```

sass output

```
a {
  filter: alpha(opacity=0.3); }

div {
  filter: alpha(opacity=0.7); }
```

libsass output

```
a {
  filter: alpha(opacity=.3); }

div {
  filter: alpha(opacity=0.7); }
```

I will shortly add more information!
