Fix for enumerable model types

There was this bug with enumerable model types in <code>ToOperation()</code>: for example, OperationModel Person[] would result in <code>{ type: "Person[]" }</code>, while it should be <code>{ type: "array", items: { "$ref": "Person" } }</code>.

As it turned out, I wrote almost the same code in the <code>CreateModelProperty()</code> method.

I combined the two into a <code>ToDataType(Type)</code> (both <code>Operation</code> and <code>ModelProperty</code> inherit from <code>DataType</code>) so it is used by both.

Edit: <code>Parameter</code> also inherits from <code>DataType</code>. Used the <code>ToDataType(Type)</code> method for creation there too in cc92b0c.