Historically, Sinatra::Response defaults to a text/html Content-Type.
However, in practice, Sinatra immediately clears this attribute after
instantiating a new Sinatra::Response object, so this default is some-
what suspect. Let's break this behavior and have Sinatra::Response
be Content-Type-less by default, and update the tests to reflect this.

Next, let's introduce a new default_content_type setting that will be
applied (instead of text/html) if the Content-Type is not set before the
response is finalized. This will be great for e.g. JSON API developers.
Let's also make it nil-able to indicate that a default Content-Type
should never be applied.

This could potentially lead to circumstances where there is content but
no Content-Type, so let's also modify Sinatra::Response to drop the body
of the response if the Content-Type is not set, and fix the errors
uncovered by that change.

Wherever Sinatra is emitting HTML, e.g. in error pages, force the
Content-Type to text/html.

Finally, clean up the error-handling logic to behave as follows:

* Set the X-Cascade header as early as possible.
* If an error block matches and returns a value, stop processing and
  return that value.
* If we have a not found or bad request error, inspect the exception (if
  any) for an error message and present it as the response body if it
  exists, or present a default error message.

The remaining logic is the same otherwise. This should make error
handlers simpler to write and behave more consistently by not polluting
the body with a default message when none may be necessary.