Fix request headers

[pepakriz]

Invalid request headers format:

POST /eet/services/EETServiceSOAP/v3 HTTP/1.1
Content-Length: 5136
User-Agent: Hele!
Host: prod.eet.cz
0: User-Agent: PHP
1: Content-Type: text/xml; charset=utf-8
2: SOAPAction: http://fs.mfcr.cz/eet/OdeslaniTrzby
3: Content-Length: 5136

see http://docs.guzzlephp.org/en/latest/request-options.html#headers