Enforce Content-MD5 of PUT requests

[gaul]

RiakCS should reject PUT requests with mismatched Content-MD5 and data. I tested using the S3-compatible API via jclouds and riakcs.net does not reject the request:

    @Test
    public void testPutBlobIncorrectMd5() throws Exception {
        String blobName = makeBlobName();
        byte[] input = TestUtils.makeRandomBuffer(/*size=*/ 1024);
        byte[] contentMd5 = Hashing.md5().hashBytes(input).asBytes();
        // Modify the first byte of the MD5
        contentMd5[0]++;
        BlobStore blobStore = blobStoreContext.getBlobStore();
        Blob blob = blobStore.blobBuilder(blobName)
                    .payload(input)
                    .contentMD5(contentMd5)
                    .build();
        thrown.expect(HttpResponseException.class);
        blobStore.putBlob(containerName, blob);
    }

Further reference:
https://twitter.com/reiddraper/status/347801372571340802

[reiddraper]

riakcs.net is running an older version of Riak CS at the moment. This was fixed in #596 and further tested in #626.