config: return `nil` on unknown option instead of raising

Returning `nil` on unknown option was default behaviour for Pry v0.12.x. In
e5556a2be8627ec3fe594c738f10422d5a1f5d43 I changed that but I am not sure if it
was intentional. This breaks plugins such as
pry-theme (https://github.com/kyrylo/pry-theme/issues/59).

Returning `nil` makes more sense, because we can write code like this:

```
Pry.config.foo ||= 123
```

...whereas as of now this is no longer possible and you would need to use
`respond_to?` to achieve the same effect.

1 files changed, 2 insertions, 2 deletions

diff --git a/spec/config_spec.rb b/spec/config_spec.rb
index db91f7cb..f1cf5ea6 100644
--- a/spec/config_spec.rb
+++ b/spec/config_spec.rb
@@ -150,8 +150,8 @@ RSpec.describe Pry::Config do
     end
 
     context "when invoked method is not an option" do
-      it "raises NoMethodError" do
-        expect { subject.foo }.to raise_error(NoMethodError)
+      it "returns nil" do
+        expect(subject.foo).to be_nil
       end
     end