Fix 500 error for xml format when method is not allowed

[304]

Problem:

All requests to Not Allowed methods are returning 500 error instead of
405 when the format is xml.

Cause:

Route definitions for not allowed methods are returning an empty string
as a response body. But the empty string fails to convert to xml and
raises InvalidFormatter error.

Solution:

Define an exception for "method is not allowed" case.
Return an error message "405 Not Allowed" instead of empty string.

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This is much better. Can you squash these two and rebase please?

[304]

@dblock, done.

[dblock]

Merged thanks.

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This introduced a behavior change that I am documenting in UPGRADING, because Grape::Exceptions::MethodNotAllowed is now raised the exception is being rescued in rescue_from :all and therefore needs to be handled explicitly. I think the following restores old behavior:

        rescue_from Grape::Exceptions::Base do |e|
          error! e.message, e.status, e.headers
        end

[dblock]

252bfd2