eIM_Tests: align RSP version number in HTTP header
The RSP version number in the x-admin-protocol header string should be set to v.2.1.0 (gsma/rsp/v2.1.0), even though the ASN.1 spec we currently use is at version 2.5.
This is to maintain compatibility with older IPAd versions and also stated explicitly in GSMA SGP.32 V2.1, section 6.1.
