clear exception context when finished reporting

otherwise, the context could be re-read, causing
unwanted overwrites and errors being reported more
than once, when they only occured once.
add a real test

note: if airbrake environment variables are
configured, this module will actually
ship errors to airbrake.io

The future goal for this module will be to send
subsequent queries which use the airbrake data API
to ensure that the errors were reported precisely
as expected, and then deleting those test records