Fix scraper name sent instead of headers'

[babolivier]

Fix a misused variable causing beamium to send headers with the scraper's name as the headers' keys.

Example configuration:

scrapers:
  scraper:
    url: http://127.0.0.1:9100/metrics
    format: prometheus
    period: 6000
    headers:
      Authorization: Basic XXXXX

[...]

Request sent without this patch:

GET /metrics HTTP/1.1
Host: 127.0.0.1:9100
scraper: Basic XXXXX

Request sent with this patch:

GET /metrics HTTP/1.1
Host: 127.0.0.1:9100
Authorization: Basic XXXXX

[d33d33]

🐙 Nice one 😄

Thanks!