Using inject breaks enumerations with proto3

When using inject alongside enumerations, the namespace of the enumeration modules is inconsistent, causing the call to [get_default](https://github.com/bitwalker/exprotobuf/blob/fc6159d7f6a0b85930b2060d93be6fe3edf25fd9/lib/exprotobuf/decoder.ex#L47) to error with a no match, coming from [here](https://github.com/tomas-abrahamsson/gpb/blob/19f29c4f7fc50763ea51f7f48d0d3c4f3ca4c262/src/gpb.erl#L1221).

Reference protobuf
```protobuf
syntax = "proto3";

message Message {
enum Status {
STATE_1 = 0;
STATE_2 = 1;
}

required int64 id = 1;
optional Status status = 2;
}

```

## Example

### Module
```elixir
defmodule MessageTest do
use Protobuf, from: Path.expand("priv/protos/message.proto"), inject: true, only: :Message
end

```

### Defs
```elixir
iex(1)> MessageTest.defs()
[{{:enum, :"Message.Status"}, [STATE_1: 0, STATE_2: 1]},
{{:msg, :Message},
[%Protobuf.Field{fnum: 1, name: :id, occurrence: :required, opts: [], rnum: 2,
type: :int64},
%Protobuf.Field{fnum: 2, name: :status, occurrence: :optional, opts: [],
rnum: 3, type: {:enum, MessageTest.Message.Status}}]}]
```

Because of mismatch of the naming (`MessageTest.Message.Status` in the field vs `:enum, :"Message.Status"` in the key), when the field is looked up in `proto3_type_default`, `lists:keyfind/3` returns false, rather than a match.

I was able to circumvent this by adding `use_module_namespace: true`, unfortunately this has the side effect of ignoring the `inject` and `only` option, as [there is no function clause that takes it into account](https://github.com/bitwalker/exprotobuf/blob/fc6159d7f6a0b85930b2060d93be6fe3edf25fd9/lib/exprotobuf.ex#L41)

When not using inject, the field defs are generated as expected

## Example

### Module
```elixir
defmodule MessageTest do
use Protobuf, from: Path.expand("priv/protos/message.proto")
end
```

### Defs
```elixir
iex(1)> MessageTest.defs()
[{{:enum, MessageTest.Message.Status}, [STATE_1: 0, STATE_2: 1]},
{{:msg, MessageTest.Message},
[%Protobuf.Field{fnum: 1, name: :id, occurrence: :required, opts: [], rnum: 2,
type: :int64},
%Protobuf.Field{fnum: 2, name: :status, occurrence: :optional, opts: [],
rnum: 3, type: {:enum, MessageTest.Message.Status}}]}]
```

I'm happy to make a pr solving this issue, but would like to get some feedback on the direction to take with this before making a pr.