firstChild does not properly work with tag name `text`

[Igor-Palaguta]

Description:

• Expected behaviour:
  element.firstChild(tag: "text") return first text node instead of element node with tag name "text"

• Actual behaviour: returns element not text node

Environment

• Package Manager:
  CocoaPods, version: 1.11.2

• Fuzi version: 3.1.3

• Xcode version: 13.4

How to reproduce:

<parent>
   <text>Some text</text>
</parent>

parent.firstChild(tag: "text")?.stringValue == " \n" instead of parent.firstChild(tag: "text")?.stringValue == "Some text"
parent.firstChild(tag: "text")?.type == .Text instead of parent.firstChild(tag: "text")?.type == .Element

parent.children(tag: "text") finds proper nodes as it checks also element type

[Igor-Palaguta]

@cezheng please have a look at #121