Environment

• CLI: 6.0.1
• Cross-platform modules: 6.0.1
• Android Runtime: 6.0.0
• iOS Runtime: 6.0.1

Describe the bug
On iOS TabStripItem font icon doesn't get the element's classes.

To Reproduce
Use the following setup (also visible in e2e/ui-tests-app -> tabs -> font icons example):

  <Tabs>
    <TabStrip>
        <TabStripItem title="First" iconSource="font://&#xF10B;" class="font-awesome font-size color"></TabStripItem>
    </TabStrip>
  ...
  </Tabs>

.font-awesome {
  font-family: "FontAwesome";
}

.font-size {
  font-size: 36;
}

.color {
  color: blue;
} 

With this setup, the font icon isn't shown on iOS.

Expected behavior
The font icon should be shown properly.

Additional Context
This alternative works because of inheritance:

  <Tabs class="font-awesome">
    <TabStrip>
        <TabStripItem title="First" iconSource="font://&#xF10B;" class="font-awesome font-size color"></TabStripItem>
    </TabStrip>
  ...
  </Tabs>

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