Updated signing v4 canonical headers to preserve quoted strings

This is way out in left field but I was working through the v4 signing instructions and noticed quoted strings were not handled correctly.

From http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html

Original

    host:iam.amazonaws.com\n
    Content-type:application/x-www-form-urlencoded; charset=utf-8\n
    My-header1:    a   b   c \n
    x-amz-date:20120228T030031Z\n
    My-Header2:    "a   b   c"\n 

Canonical

    content-type:application/x-www-form-urlencoded; charset=utf-8\n
    host:iam.amazonaws.com\n
    my-header1:a b c\n
    my-header2:"a   b   c"\n
    x-amz-date:20120228T024136Z\n

So I fixed a dupe line bug, updated tests, and added very simple logic to detect a quoted string.

Tests are green https://travis-ci.org/leetrout/boto/builds/20065380

Again, minutia, but thought it was worth fixing :weary: