Allow headers set in before block to be passed to ValidationErrors

[yairgo]

I'm using grape with grape-swagger and swagger-ui and for swagger-ui to work I need to set header values like so:

before do
  header['Access-Control-Allow-Origin'] = '*'
  header['Access-Control-Request-Method'] = '*'
end

When using parameter validations and submitting a request with invalid params, I've lost the context in which those headers were set. This causes swagger-ui to not be able to render the response.

rescue_from Grape::Exceptions::ValidationErrors do |e|
  # Headers not available here
  error!('my custom error message', 400)
end

I'm going to put in a pull request to add the headers to Grape::Exceptions::ValidationExceptions class so that I can do this

rescue_from Grape::Exceptions::ValidationErrors do |e|
  error!('my custom error message', 400, e.headers)
end

[yairgo]

dblock thanks for the quick turnaround on this.

[dm1try]

Fixed in #1063.