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
🚫	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
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.
⚠️	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 🚫 danger

[coveralls]

Coverage increased (+0.01%) to 95.858% when pulling ef6026a on fangzhu19880123:master into 53b2502 on ruby-grape:master.

[LeFnord]

thanks @fangzhu19880123 …

[LeFnord]

please add a spec