Merge context with the same key instead of replacing the old value.

[st0012]

Before:

Sentry.set_context(:character, { name: "John" })
Sentry.set_context(:character, { age: 25 })
p Sentry.get_current_scope.contexts[:character] #=> {:age=>25}

After:

Sentry.set_context(:character, { name: "John" })
Sentry.set_context(:character, { age: 25 })
p Sentry.get_current_scope.contexts[:character] #=> {:age=>25, :name=>"John"}

Closes #1619