Don't use default parameters for booleans as they'll override default
settings on the site. For instance, when automatic conversion to text is
selected on Mandrill, not including the auto_text parameter explicitly
in your data structure locally will cause false to be sent, overriding
the default (if it's set to true). So, mark them as omitempty instead.
Merge pull request #1 from jefferai/master

Don't use default parameters for booleans as they'll override default settings on the site.