Fix user metadata extraction

In non-MP PUT object, only x-amz-meta-* headers were stored as metadata.
This commit adds some standard headers to metadata as MP upload.

In MP upload, x-amz-meta-* header names were stored in capital, e.g. X-Amz-Meta-*.
This commit changes these names to all lower cases, e.g. x-amz-meta-*.
Add boto test for object metadata
Add response XML and boto workaround (preceding slash)
Replace all user metadata

Not only x-amz-meta-* but normal HTTP headers, e.g. Content-Encoding
Add boto test for PUT Object Copy with zero-length
Merge pull request #511 from basho/bugfix/handle-object-metadata-headers

capitalization of x-amz-meta- header on multipart upload.