Fix #head and #get helpers on Relation using the default method

Specifying the method to #call without a body is ambiguous, so that
method's documentation has been updated to reflect actual behavior.

A user could theoretically set up a Relation with a default method of POST for an endpoint that expects a `method` value in a body being serialized from a hash.  I'm not sure there's a way to differentiate that case from someone trying to specify a body-less method type in the options hash that doesn't violate the principle of least surprise in at least some cases.