Fix for Issue #3

Hi,

Hopefully I've done this pull request malarkey right.  If not, please give me a shout.

Lines 28-38 of EngageUrlBuilder class have changed (the diff seems to suggest more!?!?)

Hope this helps,
Franz.
-----------

Issue https://github.com/TroyGoode/EngageNet/issues/3

Issue was case-sensitive string comparison when checking for HTTPS being
"ON".  Changed to case-insensitive comparison.