Check for @values in StripeObject#method_not_found

If a StripeObject is being deserialized by psych, @values.has_key? is
called before @values is initialized which prevents proper
deserialization from occurring. Checking for existence first resolves
the issue.
Merge pull request #122 from mcolyer/fix-psych-deserialization

Check for `@values` in StripeObject#method_not_found.

This is necessary because we check for `@values` in `respond_to_missing?` now, but this isn't necessarily always set. For example, in deserializing from YAML, Psych creates the object using `allocate` and not `new`, which ends up skipping any initializing of instance variables. See #122 for more details.