NbLayoutHeaderComponent set fixed=false also if subheader=false

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

* [ X ] bug report
* [ ] feature request

### Issue description
Fixed is setted to false also if subheader=false

**Current behavior:**

set nb-layout-header in this way:
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**Expected behavior:**

NbLayoutHeaderComponent have to check subheader value before set to false fixed.
**Steps to reproduce:**

Checkout ngx-admin and then add to @theme/layouts/components/one-column.layout.html
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**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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