Store helper proxies instead of raw view contexts

[haines]

Any time we want to use Draper::ViewContext.current to access helper methods, we create a helper proxy to send the helpers because things like html_escape are private. Each decorator class gets its own stateless HelperProxy

This pull request changes this behaviour so that instead of having one proxy per decorator class, we create a proxy when we store the view context. All decorators then use this same helper proxy, which means that in tests we can do something like this

it "stubs a helper" do
  helper.stub content_tag: "hello"
  expect(decorator.h.content_tag).to eq "hello"
end

which is not currently possible because helper and decorator.h are different objects (#436).

I also made HelperProxy#method_missing define a method on the HelperProxy class so that future calls to the helper don't go through method_missing again, à la #89.

[steveklabnik]

https://travis-ci.org/drapergem/draper/jobs/4345115 :/

[haines]

Oops. No idea why this doesn't work on JRuby for Rails 3.x, but in any case looks like Rails 4 won't delegate non-public methods, so I'll change it to do an explicit send.

[haines]

Sweet, it works now, after sorting out an incompatibility which I assume arose from an update to a JDBC driver (ugh!).

[carloslopes]

Change tested and working here guys