Avoid call to rb_string_value_cstr() which doesn't handle binary strings properly

I believe this is all that's necessary to resolve #32 

I've done some manual testing to validate that we're sending the right bytes through Kafka, in the integration test I printed out the md5 of the byte string going in, and then printed the md5 of the buffer that a Kafka consumer (using the Java Kafka library) reads off:

**producer side**
```
Hermann::Lib::Producer
  #push_single
    with binary data
hash: a3e3316785491f09d90fd57b2d412437
      should return
```

**consumer side**
```
"\n+AuOzetQrTrdwSY14ig7I_1oUwjp3DvTx3YWhSTGD4Fo\x12\x192014-09-10T00:18:47-07:00\x1A,\n\x06scream\x12\x0Emissing_device\x1A\x12flexd-today-0-app0\"\t\n\astarted*(\b\x00\x12$009f0305-b50a-455d-b137-e52b45f674aa*(\b\x01\x12$53c0d817-d94b-4b7a-9a58-95fe8cec4333"
Hash: a3e3316785491f09d90fd57b2d412437
Read: 213 from Kafka
```