[wip] Attempts to fix: https://github.com/bluelabsio/s3-stream/issues/2

[hderms]

What

Add a metadata class to the relevant signatures so that we can set contentType in the initial HTTP request to amazon. Metadata is being passed instead of just contentType to make way for more options to be set in the future

Why

• Setting the ContentType can be important for various application use-cases and having to perform subsequent HTTP requests to set it is not in the spirit of efficiency that a streaming library should strive for.
• Allow setting Content-Type on Upload #2