Added srcset support (#18)

* added src set as a parsed attribute

* added tests

* changed to /a

* added multiple attribute handling

* added better comment

* made snapshot ignore invalid input as if it is invalid input in the original DOM, it should stay invalid in the recreated DOM

* added extra absolute test case

* code style

* addressed comments

src/snapshot.ts

@@ -85,7 +85,36 @@ export function absoluteToStylesheet(cssText: string, href: string): string {
   });
 }
 
+function getAbsoluteSrcsetString(doc: Document, attributeValue: string) {
+  if(attributeValue.trim() === "") {
+    return attributeValue
+  }
+
+  const srcsetValues = attributeValue.split(",")
+  // srcset attributes is defined as such:
+  // srcset = "url size,url1 size1"
+  const resultingSrcsetString = srcsetValues.map((srcItem) => {
+       // removing all but middle spaces
+       const trimmedSrcItem = srcItem.trimLeft().trimRight()
+       const urlAndSize = trimmedSrcItem.split(" ")
+       // this means we have both 0:url and 1:size
+       if (urlAndSize.length === 2) {
+         const absUrl = absoluteToDoc(doc, urlAndSize[0])
+         return `${absUrl} ${urlAndSize[1]}`
+       } else if(urlAndSize.length === 1){
+         const absUrl = absoluteToDoc(doc, urlAndSize[0])
+         return `${absUrl}`
+       }
+       return ""
+  }).join(',')
+
+  return resultingSrcsetString
+}
+
 function absoluteToDoc(doc: Document, attributeValue: string): string {
+  if (attributeValue.trim() === ""){
+    return attributeValue
+  }
   const a: HTMLAnchorElement = doc.createElement('a');
   a.href = attributeValue;
   return a.href;
@@ -132,6 +161,8 @@ function serializeNode(
         // relative path in attribute
         if (name === 'src' || name === 'href') {
           attributes[name] = absoluteToDoc(doc, value);
+        } else if (name == 'srcset') {
+          attributes[name] = getAbsoluteSrcsetString(doc, value)
         } else if (name === 'style') {
           attributes[name] = absoluteToStylesheet(value, location.href);
         } else {