Icon won't allow config on enabled tab of NbRouteTabsetComponent

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request

### Issue description
According to https://github.com/akveo/nebular/pull/1935 (which closed https://github.com/akveo/nebular/issues/1323 ) , components which use icons should be accepting an icon config object (instead of just a string), however this commit https://github.com/akveo/nebular/pull/1935/commits/13d74cf79adcf77f4bf633e1559a315d9d59fad2 is missing the change on the `enabled` tab template, so it only works for disabled tabs.

The fix should be easy, just change the line:
https://github.com/akveo/nebular/blob/ac50daebc6b3cbae52f593bd2c878d7cacb94246/src/framework/theme/components/route-tabset/route-tabset.component.ts#L105

to bind to the `[config]` property instead of `[icon]`, as done on the `disabled` tab code a few lines above it on the specified commit:
https://github.com/akveo/nebular/blob/ac50daebc6b3cbae52f593bd2c878d7cacb94246/src/framework/theme/components/route-tabset/route-tabset.component.ts#L91

### Other information:

Tested on Angular 9 and nebular 5.0, but the problem is quite obvious that it's still there on `master`, and should be an easy fix (I really don't have time to make a pull request right now or I'd gladly do it)