Export KeyInfo.GetSSEHeaders so users can more easily generate SSE-C headers

[donatello]

This adds a method for users to more simply generate and set server-side-encryption headers.

[harshavardhana]

Lets keep it map[string][]string if we are really going to export it.

[donatello]

No, doing it this way is the cleaner interface for users when adding these headers to another existing set of headers. Compare:

// In the scenarios below, metadata is of type map[string][]string as this is the type 
// passed to put-object requests.

// 1. When header value is just a string:
key := NewSSEInfo("blahblah", "")
for k, v := range key.GetSSEHeaders() {
     metadata[k] = append(metadata[k], v)
}

// 2. When header value is a []string:
key := NewSSEInfo("blahblah", "")
for k, v := range key.GetSSEHeaders() {
     // Either we extract v[0] alone - which means implementation could have to change in future, or...
     metadata[k] = append(metadata[k], v...)
}

The point here is that there is no reason to making the return type a slice - there should be no option for user to append extra stuff for these sse-c headers.

[harshavardhana]

Hmm also Go doesn't allow immutable maps. Makes sense..

[donatello]

Any other reviews?