Remove NilAway detections for 'client' package

$ nilaway ./client
/Users/ash/src/gitlab/gitlab-development-kit/gitlab-shell/client/gitlabnet.go:120:59: error: Potential nil panic detected. Observed nil flow from source to dereference point:
	-> client/gitlabnet.go:136:23: result 0 of `Do()` lacking guarding; passed as arg `resp` to `parseError()`
	-> client/gitlabnet.go:120:59: function parameter `resp` accessed field `StatusCode`

/Users/ash/src/gitlab/gitlab-development-kit/gitlab-shell/client/gitlabnet.go:120:59: error: Potential nil panic detected. Observed nil flow from source to dereference point:
	-> client/gitlabnet.go:170:23: result 0 of `Do()` lacking guarding; passed as arg `resp` to `parseError()`
	-> client/gitlabnet.go:120:59: function parameter `resp` accessed field `StatusCode`

/Users/ash/src/gitlab/gitlab-development-kit/gitlab-shell/client/httpclient_test.go:73:54: error: Potential nil panic detected. Observed nil flow from source to dereference point:
	-> strings/strings.go:242:10: literal `nil` returned from `genSplit()` in position 0
	-> strings/strings.go:312:45: result 0 of `genSplit()` returned from `Split()` in position 0
	-> client/httpclient_test.go:73:54: result 0 of `Split()` sliced into

(Same nil source could also cause potential nil panic(s) at 1 other place(s): client/httpclient_test.go:71:28.)