Ensure that ssl option is read as expected

[evz]

Hello there! I was attempting to mock out the calls to the Segment API for the purposes of load testing and found that if you set the ssl option to false (which is what you would expect to work), the way that the code was written in the Transport class, meant that it was always being set to the default (since options[:ssl] evaluates to false, using the ||= operator always flipped it back to true aka the default).

This attempts to fix that issue by making it so that setting ssl to anything other than true will cause it to be set to false. I expect that this will more than likely be fine since this isn't really even documented as an option.