Extend :cascade to work with all versioning strategies

[dieb]

The :cascade option allows preventing X-Cascade to be returned on
response headers. For Rails users, this is useful because your
API exceptions don't get rendered/handled by Rails, but actually sent to
users. This is particularly useful if you want your user to receive an
error exception in the same format your API supports, and not an
unexpected HTML page.

A more in-depth explanation of :cascade was moved into the code and the
overview was inserted on the main README on the exceptions section.

This commit also changes the order in which versioning strategies are
presented. The order is now consistent with the order in which the
strategies were first presented. It also presents the default one,
before introducing non-defaults.

[dieb]

As noted on #340, mounting Grape inside rails will likely get exceptions rendered by rails, which for some users may not be pleasant.

By setting :cascade => false, the API can now disable this behaviour for requests to the API path.

[dblock]

I merged this with a small change: deleting the X-Cascade header instead of setting it to blank. If I understand this correctly, it has the same effect, but please correct me if I am wrong.

6b2e0cb

[dieb]

I first wrote is as deleting the header but for some reason I changed it to unsetting but I don't remember why :-( I'll try to remember. Hahaha

[dieb]

BTW Did you run rspec with the header deleted? It might break the second test case I prepared.

[dblock]

I did fix the test ;)