Custom roles do not work in HorizontalHeaderView

Binding a property of the delegate of a `HorizontalHeaderView` to a custom role of a `QAbstractTableModel` doesn't successfully fetch the relevant data from `headerData()`. 

Note that when running the example below:

• Header text says "This is display table data"
• Table text says "This is custom table data"
• Terminal shows `Display header data requested`
• Terminal shows `Custom table data requested`

To reproduce bug:

• Change the line marked `// BUG HERE` to `text: model.my_custom_role`.

Expected behaviour:

• Header text shows "This is custom data", just like the table itself. 
• Terminal shows `Custom header data requested`

Actual behaviour:

• Header text blank
• Error message in terminal: "Unable to assign [undefined] to QString"
• Terminal does not show `Custom header data requested`