Correct handling of unknown proto3 enum values

[judah]

Currently proto-lens returns an error when decoding unknown enum values. It should instead accept and preserve such values.

Quoting the proto3 docs:

During deserialization, unrecognized enum values will be preserved in the message, though how this is represented when the message is deserialized is language-dependent. In languages that support open enum types with values outside the range of specified symbols, such as C++ and Go, the unknown enum value is simply stored as its underlying integer representation. In languages with closed enum types such as Java, a case in the enum is used to represent an unrecognized value, and the underlying integer can be accessed with special accessors. In either case, if the message is serialized the unrecognized value will still be serialized with the message.

[judah]

Fixed by #137.