Extend CssParser quote support

Updates the CssParser.ParseBackgroundImageProperty method to correctly
support values surrounded with double quotes.

Author: cyotek

Source/HtmlRenderer/Parse/CssParser.cs

@@ -553,9 +553,9 @@ private static string ParseBackgroundImageProperty(string propValue)
                 if(endIdx > -1)
                 {
                     endIdx -= 1;
-                    while (startIdx < endIdx && (char.IsWhiteSpace(propValue[startIdx]) || propValue[startIdx] == '\''))
+                    while (startIdx < endIdx && (char.IsWhiteSpace(propValue[startIdx]) || propValue[startIdx] == '\'' || propValue[startIdx] == '"'))
                         startIdx++;
-                    while (startIdx < endIdx && (char.IsWhiteSpace(propValue[endIdx]) || propValue[endIdx] == '\''))
+                    while (startIdx < endIdx && (char.IsWhiteSpace(propValue[endIdx]) || propValue[endIdx] == '\'' || propValue[endIdx] == '"'))
                         endIdx--;
 
                     if (startIdx <= endIdx)