Headers parsing fix

The s3curl was parsing headers incorrectly, so the header-values were
converted to lowercase before sending for HMAC-signature calculation.

After the fix, the following examples work correctly (please adjust buckets/keys):
    s3curl.pl --id=xx --copySrc /yy/zz -- http://bucket.s3.amazonaws.com/newKey \
	-H "x-amz-copy-source-if-modified-since: Tue, 20 May 2014 07:50:15 -0700" -v
    s3curl.pl --id=xx --createBucket -- http://bucket.s3.amazonaws.com \
	-H "x-amz-grant-full-control: uri=http://acs.amazonaws.com/groups/global/AllUsers" -v

Also fixed the debug-message with the curl-invocation, so that the
parameters are quoted if necessary.