Add support for 'brackets' collection format

[korstiaan]

Although not in the official spec, it is support by swagger-ui. The multi format (foo=a&foo=b) doesn't work with rack base apps (like rails). Rack wants it to look like foo[]a=&foo[]=b, which is exactly what the brackets format specifies.

[coveralls]

Coverage remained the same at 97.644% when pulling 99d993a on korstiaan:add_brackets_collection_format into bb68d90 on ruby-grape:master.

[coveralls]

Coverage remained the same at 97.644% when pulling bfe2dd0 on korstiaan:add_brackets_collection_format into bb68d90 on ruby-grape:master.

[LeFnord]

thanks @korstiaan

[viral810]

@LeFnord Am I missing anything here? seems like this doesn't do anything except the fact that you can use collectionFormat: 'brackets' in documentation hash.

For example,

if param_type_is_array?(param_type)
            options[:is_array] = true
            name += '[]' if array_use_braces? # Shouldn't this also check the collectionFormat from the documentation?
          end

cc: @korstiaan @magni-