clients: Fix request with headers set

[gruas]

bombardier was failing to connect to the server if custom headers were set. This happened because the Host header was being overwritten during request creation. By reordering the operations (first set the headers, then the Host header) we can avoid this problem.

Example problem:

$ go run . -H 'Content-Type: application/json' -m POST http://localhost:8000/anything -p r -t 1s -n 1
Statistics        Avg      Stdev        Max
  Reqs/sec      1372.12       0.00    1372.12
  Latency       10.00us     0.00us    10.00us
  HTTP codes:
    1xx - 0, 2xx - 0, 3xx - 0, 4xx - 0, 5xx - 0
    others - 1
  Errors:
    dial tcp: missing address - 1
  Throughput:       0.00/s