Generic exceptions have an incorrect mimetype: text/html
== Reproduction steps: ==
Write a read method for an API
Have this method raise a generic Exception.
Retrieve this URL in a browser.
== Excepted result: == You can read the exception, formatted as text/plain.
== Actual result: == The exception is all smashed together, because it defaulted to text/html.
== Notes: == A patch is attached, based on Piston 0.2.2.