Fixed underlining when no explicit foreground color is set (#37)

* Underlining doesn’t require specifying a foreground color

Made `color` optional when creating an underline element. In addition, if the `underlineColor` attribute is present, it will be preferred over `foregroundColor` (if any). If neither is specified, the underline color will not be output explicitly.

* No longer explicitly look for `foregroundColor` when setting color on an underline

When no underline color is explicitly specified on the <w:u> element, Word already falls back to the text color. So, there’s no reason for DocX to perform this check as well.

DocX/AttributeElements.swift

@@ -40,9 +40,12 @@ extension Dictionary where Key == NSAttributedString.Key{
             attributesElement.addChildren(self.outlineProperties(strokeWidth: strokeWidth, font:font))
         }
 
-        if let style=self[.underlineStyle] as? Int, let color=self[.foregroundColor] as? NSColor{
+        if let style=self[.underlineStyle] as? Int {
+            // If the `underlineColor` attribute is present, use that as the color
+            let underlineColor = self[.underlineColor] as? NSColor
+
             let underline=NSUnderlineStyle(rawValue: style)
-            attributesElement.addChild(underline.underlineElement(for: color))
+            attributesElement.addChild(underline.underlineElement(for: underlineColor))
         }
 
         if let backgroundColor=self[.backgroundColor] as? NSColor{

DocX/NSUnderlineStyle+Elements.swift

@@ -41,9 +41,17 @@ extension NSUnderlineStyle{
         return val
     }
 
-    func underlineElement(for color:NSColor)->AEXMLElement{
-        let colorString=color.hexColorString
-        return AEXMLElement(name: "w:u", value: nil, attributes: ["w:color":colorString, "w:val":self.elementValue])
+    func underlineElement(for color:NSColor?)->AEXMLElement{
+        // Always add the underline value, which determines the underline style
+        var attributes = ["w:val": self.elementValue]
+
+        // If color is specified, include that too
+        if let color = color {
+            let colorString = color.hexColorString
+            attributes["w:color"] = colorString
+        }
+
+        return AEXMLElement(name: "w:u", value: nil, attributes: attributes)
     }
 
     var strikeThroughElement:AEXMLElement{