Provide a #to_h method for Unleash::Context

[tmaier]

Describe the feature request

The instance of context should be able to be converted to a hash.
The ruby naming convention is a #to_h method to do so.

Background

Sometimes, one wants to re-use the context set. e.g. to report it to Sentry or any other exception tracker.

Solution suggestions

Add a to_h method to Unleash::Context

[daveleek]

Thank you for the suggestion and the PR @tmaier, we'll have a look!