fix case in server_create parameters
fix case in server_create parameters

Vultr seems to be case sensitive.  This would silently fail: the server will be
created with no errors, but the relevant parameters will have been ignored.

(I haven't checked whether this problem applies to other API calls.  You may
want to give them a look.)

dopry/python-vultr,spry-group/python-vultr,getlantern/python-vultr