aws/request: make-operation->request: Use lowercase for authorization.

aws/request: make-operation->request: Use lowercase for authorization.

When we use the symbol 'Authorization as the name of the header, Guile
complains "Bad request: Bad value for header Authorization".

We don't want or need Guile to make this judgement.  By using
'authorization we avoid this problem.

* aws/request.scm (make-operation->request)[new-headers]: Use
lowercase for authorization header name.