Do not fail download if content length is unknown (#977)

Fixes ForgeGradle crashing with cryptic `Cannot resolve external dependency net.minecraftforge:accesstransformers:1.0.+` errors. The real exception was suppressed.

Partial backport of #905 to ForgeGradle 3.

Author: Jonathing

src/common/java/net/minecraftforge/gradle/common/util/Utils.java

@@ -426,7 +426,7 @@ private static boolean downloadFile(URLConnection con, File output) throws IOExc
                 read = IOUtils.copy(stream, out);
             }
 
-            if (read != len) {
+            if (len != -1 && read != len) {
                 output.delete();
                 throw new IOException("Failed to read all of data from " + con.getURL() + " got " + read + " expected " + len);
             }