response.headers['my-header'] should case insensitive

[krames]

Recently fog has recently been impacted several times by the case sensitivity of response.headers. It seems to be a valid expectation that this would be case insensitive.

I propose that we deprecate response.get_header and update the header implementation. This related to issue #363.

[geemus]

I have some concern about performance implications for this. case insensitive lookup is inherently slower, probably a small amount in some cases, but these lookups happen kind of a lot. Something to think about anyway.