Apparently, as outlined in basho/webmachine#117, the header is actually supposed to be base64-encoded. We should correct this.
See also basho/webmachine#116
decode 'Content-MD5' checksum as base64-encoded string.
in the RFC the header is defined as:
Content-MD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>