Removed unnecessary duplicate XMLHeader.init

Set defaults for params of XMLHeader.init
Made XMLHeader.init public for use outside of module.

Author: kyleishie

Sources/XMLCoding/XMLStackParser.swift

@@ -24,11 +24,7 @@ public struct XMLHeader {
     /// indicates whetehr a document relies on information from an external source.
     var standalone: String? = nil
 
-    init(version: Double? = nil) {
-        self.version = version
-    }
-    
-    init(version: Double?, encoding: String?, standalone: String? = nil) {
+    public init(version: Double? = nil, encoding: String? = nil, standalone: String? = nil) {
         self.version = version
         self.encoding = encoding
         self.standalone = standalone