Added a test for uploadPart with max size.

The max size for a part upload should be 4GB, but jdk 6 doesn't
allow us to make 2GB or bigger uploads. Since glacier requires the
parts to have a size power of two, the max size we can achieve per
part is 1GB.
Now the testUploadPartMaxSize test is faster.

The testUploadPartMaxSize required the mockwebserver to
read the whole payload. The test took ~10-30 seconds with 1GB
payload and ~1-2 minutes with 4GB payload.

With this small trick the test is much faster.