[Issue 1163] Add additional tests

[dpmott]

[Issue 1163] Add additional tests

EDIT: This also fixes issue https://github.com/ngx-translate/core/issues/1176

• Rolled back changes to translate.directive.ts from commit 24b7b2b8cb716adcb25905df9d64e5da43d41838 (but keeping tests)
• Added additional tests.
• Issue 1163 appears to have actually been fixed by 1167/1164, as evidenced by the newly added tests.

[judomu]

We're having the same issue and so far I moved from directive to pipes to mitigate the issue, but it would be perfect to be able to use directive in all cases :)

[tiaguinho]

@ocombe can you take a look into this?