support multiple merge fields

[fangzhu19880123]

I want to use like this

expose :config1, merge: true, using: ApplicationEntity
expose :config2, merge: true, using: ApplicationEntity

but I must use like this

expose :config1, merge: true, using: ApplicationEntity
expose :config2, merge: ->(k, v1, v2) { v2 }, using: ApplicationEntity

[grape-bot]

	1 Error
:no_entry_sign:	One of the lines below found in CHANGELOG.md doesn’t match the expected format. Please make it look like the other lines, pay attention to periods and spaces.

	2 Warnings
:warning:	Unless you’re refactoring existing code, please update CHANGELOG.md.
:warning:	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

* [#258](https://github.com/ruby-grape/grape-entity/pull/307): Allow exposures to call methods defined in modules included in an entity [@robertoz-01](https://github.com/robertoz-01).

Here's an example of a CHANGELOG.md entry:

* [#310](https://github.com/ruby-grape/grape-entity/pull/310): Support multiple merge fields - [@fangzhu19880123](https://github.com/fangzhu19880123).

Generated by :no_entry_sign: danger

[coveralls]

Coverage increased (+0.01%) to 95.858% when pulling ef6026a1d33aff4c7d3bfef8e7f208821e8c8c79 on fangzhu19880123:master into 53b250299855dd8be5db4c14b31a9c5d663b42ef on ruby-grape:master.

[LeFnord]

thanks @fangzhu19880123 …