Add #only to whitelist keys

[ryansch]

In my integration request specs, I sometimes want to find out if a json response contains objects with specific ids. With the existing interface I have to go through and explicitly blacklist all of the other keys. I instead added a whitelist with the #only method.

Example:

# include_json
%([{"id":1,"json":"spec"}]).should include_json(%({"id":1})).only(:id)

# be_json_eql
%({"id":1,"json":"spec"}).should be_json_eql(%({"id":1})).only(:id)

The actual json responses I'm comparing have far more keys.

[charlierudolph]

This can actually be done with the current interface

First example:

%([{"id":1,"json":"spec"}]).should be_json_eql(1).at_path("0/id")

Second example:

%({"id":1,"json":"spec"}).should be_json_eql(1).at_path("id")

[ryansch]