Do not perform entity filtering on msgid.

These aren't presented, should never have markup and it breaks matching.

ASAP @yegg @bsstoner 

To illustrate the issue, currently:

```po
msgid "Hello &#39;%s&#39;"
msgstr "Hallo &#39;%s&#39;"
```

...the token Hello '%s' won't be matched. This is fixed with this change:

```po
msgid "Hello '%s'"
msgstr "Hallo &#39;%s&#39;"
```