Add expose_nil option

[b-boogaard]

Addresses #216 and #10

I read through both of the issues to help guide naming, default behavior etc.

Some basic high level details

• expose_nil defaults to true
• is implemented as an unless condition
• the option is transformed as part of Exposure.compile_conditions

Currently rubocop is failing because my new tests make the describe block too long. I'm not sure how you all typically handle something like this so just let me know what I should do.

Running RuboCop...
Inspecting 38 files
...............................C......

Offenses:

spec/grape_entity/entity_spec.rb:6:1: C: Metrics/BlockLength: Block has too many lines. [1625/1496]
describe Grape::Entity do ...
^^^^^^^^^^^^^^^^^^^^^^^^^

38 files inspected, 1 offense detected
RuboCop failed!

[coveralls]

Coverage increased (+0.03%) to 95.646% when pulling 5feb848 on b-boogaard:add-expose-nil-opt into 4b0ea7f on ruby-grape:master.

[DmitryTsepelev]

@b-boogaard you can increase Metrics/BlockLength: max value in the rubocop_todo.yml :) or move the whole context to the different spec file

[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.

* Adds expose_nil option - [@b-boogaard](https://github.com/b-boogaard)

* [#288](https://github.com/ruby-grape/grape-entity/pull/288) Fix wrong argument exception when &:block passed to the expose method  - [@DmitryTsepelev](https://github.com/DmitryTsepelev).

* [#291](https://github.com/ruby-grape/grape-entity/pull/291) Refactor and simplify various classes and modules  - [@DmitryTsepelev](https://github.com/DmitryTsepelev).

Generated by 🚫 danger

[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.

* [#291](https://github.com/ruby-grape/grape-entity/pull/291) Refactor and simplify various classes and modules  - [@DmitryTsepelev](https://github.com/DmitryTsepelev).

* [#288](https://github.com/ruby-grape/grape-entity/pull/288) Fix wrong argument exception when &:block passed to the expose method  - [@DmitryTsepelev](https://github.com/DmitryTsepelev).

Generated by 🚫 danger

[b-boogaard]

@DmitryTsepelev I updated the rubocop_todo.yml. It seemed a little presumptuous to do without a green light on my first PR. :)

[LeFnord]

thanks @b-boogaard

[b-boogaard]

Sure thing @LeFnord