Simplest behavior: null -> exception, like the other params.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/csv/trunk@1578502 13f79535-47bb-0310-9956-ffa450edef68

Author: garydgregory

src/main/java/org/apache/commons/csv/CSVParser.java

@@ -183,7 +183,7 @@ public static CSVParser parse(final String string, final CSVFormat format) throw
      * @param url
      *            a URL. Must not be null.
      * @param charset
-     *            the charset for the resource. If {@code null}, use {@code UTF-8}.
+     *            the charset for the resource. Must not be null.
      * @param format
      *            the CSVFormat used for CSV parsing. Must not be null.
      * @return a new parser
@@ -194,10 +194,10 @@ public static CSVParser parse(final String string, final CSVFormat format) throw
      */
     public static CSVParser parse(final URL url, final Charset charset, final CSVFormat format) throws IOException {
         Assertions.notNull(url, "url");
+        Assertions.notNull(charset, "charset");
         Assertions.notNull(format, "format");
 
-        return new CSVParser(new InputStreamReader(url.openStream(),
-                             charset == null ? Charset.forName("UTF-8") : charset), format);
+        return new CSVParser(new InputStreamReader(url.openStream(), charset), format);
     }
 
     // the following objects are shared to reduce garbage