Fix to issue #180 to handle null contentType

If the content type is null, the call for .toString on it will cause an NPE. This fixes this issue by wrapping the call in an Option, then mapping toString or falling back to null. getCharSetFromContentType handles null by returning ISO-8859-1

ivy/src/main/scala/sbt/internal/librarymanagement/ivyint/GigahorseUrlHandler.scala

@@ -51,7 +51,7 @@ class GigahorseUrlHandler extends AbstractURLHandler {
 
         if (checkStatusCode(url, response)) {
           val bodyCharset =
-            BasicURLHandler.getCharSetFromContentType(response.body().contentType().toString)
+            BasicURLHandler.getCharSetFromContentType(Option(response.body().contentType()).map(_.toString).orNull)
           Some(
             new SbtUrlInfo(true,
                            response.body().contentLength(),