Added a fingerprint parameter to set_metadata request, since without this property the request with 'CONDITION_NOT_MET' status.
Switched places between the optional arguments for the set_metadata request on google provider to support older ruby versions.
1. Added the fingerprint parameter to the set_metadata mock definition
2. Added comments to set_metadata to clarify how the fingerprint parameter can be generated.
Merge pull request #2678 from leonidlm/master

[google|compute] Fixing the set_metadata request