Fix garbled request input from file

Buffer a copy of `scanner.Bytes()` when appending to a slice of requests.
Otherwise the data may be overwritten in subsequent loops. This results in
garbled requests, without truncated output and sometimes random characters.
This may not be apparent when a small number of requests are used.

Ref: http://golang.org/pkg/bufio/#Scanner.Bytes
```
Bytes returns the most recent token generated by a call to Scan. The
underlying array may point to data that will be overwritten by a subsequent
call to Scan. It does no allocation.
```

This fixes the tests errors like:
```
2013/10/17 15:26:28 Starting file reply2013/10/17 15:26:28 Can not parse request =b&c=d
1382023587787798594
POST /test HTTP/1.1
Host: localhost:50030
User-Agent: Mozilla/5.0 (Unknown; Linux x86_64) AppleWebKit/534.34 (KHTML, like Gecko) PhantomJS/1.9.1 Safari/534.34
Content-Length: 7
Accept: */*
Accept-Language: en-GB,*
Cookie: test=value
Referer: http://localhost/test
X-Forwarded-For: 1.1.1.1, 2.2.2.2, 3.3.3.3
X-Fo malformed HTTP request "=b&c=d"
2013/10/17 15:26:28 Can't parse request...:malformed HTTP request "=b&c=d"
```

--

Includes a new test. Although it was masked by the failing limit test.

Might fix #34, but I can't tell whether it's exactly the same thing.