fix issue #21 charset detect error

webmagic-core/src/main/java/us/codecraft/webmagic/utils/UrlUtils.java

@@ -2,6 +2,7 @@ package us.codecraft.webmagic.utils;
 
 import org.apache.commons.lang3.StringUtils;
 
+import java.nio.charset.Charset;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -98,15 +99,17 @@ public class UrlUtils {
         return stringBuilder.toString();
     }
 
-    private static final Pattern patternForCharset = Pattern.compile("charset=([^\\s;]*)");
+    private static final Pattern patternForCharset = Pattern.compile("charset\\s*=\\s*['\"]*([^\\s;'\"]*)");
 
     public static String getCharset(String contentType) {
         Matcher matcher = patternForCharset.matcher(contentType);
         if (matcher.find()) {
-            return matcher.group(1);
-        } else {
-            return null;
+            String charset = matcher.group(1);
+            if (Charset.isSupported(charset)) {
+                return charset;
+            }
         }
+        return null;
     }
 
 }