Add more comprehensive byte serving specs

Currently partial content test accounts only for one Range header format:
```
Range: bytes=$start-$end
```

RFC specifies how the server should respond if either the start or end is missing. I've added two specs to comply to the standard: 

When the start is missing the server should respond with the last $end number of bytes of the file.

```
Range: bytes=-10
```

This header is requesting the last 10 bytes of the file.

When the end is missing, the server should respond with all bytes from $start to the end of the file, for      example:

```
Range: bytes=4-
```
Means that we should serve all bytes of the file starting at index 4.