Use Constants.CHARACTER_ENCODING in tests

Change-Id: Iba9fc991aee54fcb8b0dc5e5841da24c2d54dfc2
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/SmartClientSmartServerTest.java

 					throws IOException, ServletException {
 				final HttpServletResponse r = (HttpServletResponse) response;
 				r.setContentType("text/plain");
-				r.setCharacterEncoding("UTF-8");
+				r.setCharacterEncoding(Constants.CHARACTER_ENCODING);
 				try (PrintWriter w = r.getWriter()) {
 					w.print("OK");
 				}

org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/ProxyConfigTest.java

 import java.util.List;
 import java.util.Map;
 
+import org.eclipse.jgit.lib.Constants;
 import org.junit.Before;
 import org.junit.Test;
 
 			while ((length = inputStream.read(buffer)) != -1) {
 				result.write(buffer, 0, length);
 			}
-			return result.toString("UTF-8");
+			return result.toString(Constants.CHARACTER_ENCODING);
 		}
 	}
 }