Support using matcher in examples without a string

[tjsousa]

Fixes an issue when trying to use the matchers in one line expectations without a string description.

This fix has been tested with rspec-has to write it like this:

it { has.to trigger_statsd_increment("metric") } 

Original error:

When you call a matcher in an example without a String, like this:

specify { expect(object).to matcher }

or this:

it { is_expected.to matcher }

RSpec expects the matcher to have a #description method. You should either
add a String to the example this matcher is being used in, or give it a
description method. Then you won't have to suffer this lengthy warning again.

[wvanbergen]

@tjsousa thanks for contributing. This looks good! However, before I can merge you'll need to sign Shopify's (pretty standard) contributor agreement.

[tjsousa]

Hi @wvanbergen, signed it. Anything else I would have to do?

[wvanbergen]

All good now, merging!