re-use and freeze string values that never change.
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>

closes #97
Merge pull request #149 from seancribbs/content-md5

decode 'Content-MD5' checksum as base64-encoded string