Verify content-length for Net::HTTP responses.

Some downloads from Amazon S3 can be truncated, and Net::HTTP
does not check response body size against the content-length
specified in the response header. This change adds such a check,
which can be toggled on or off (on by default) in the AWS.config.
Update AWS::Core documentation for verifying content length.

Adds matching documentation for new verify content length option
to core.rb